docs: update some straggling Documentation/arm references

The Arm documentation has moved to Documentation/arch/arm; update the
last remaining references to match.

Cc: Dmitry Torokhov <[e-mail]>
Cc: Chen-Yu Tsai <[e-mail]>
Cc: Samuel Holland <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Acked-by: Jernej Skrabec <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Uwe Kleine-König <[e-mail]> # for pwm
Signed-off-by: Jonathan Corbet <[e-mail]>

MAINTAINERS

···
 B:	mailto:[e-mail]
 C:	irc://irc.libera.chat/linux-exynos
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/krzk/linux.git
-F:	Documentation/arm/samsung/
+F:	Documentation/arch/arm/samsung/
 F:	Documentation/devicetree/bindings/arm/samsung/
 F:	Documentation/devicetree/bindings/hwinfo/samsung,*
 F:	Documentation/devicetree/bindings/power/pd-samsung.yaml
···
 L:	[e-mail]
 L:	[e-mail]
 S:	Orphan
-F:	Documentation/arm/omap/dss.rst
+F:	Documentation/arch/arm/omap/dss.rst
 F:	drivers/video/fbdev/omap2/
 
 OMAP FRAMEBUFFER SUPPORT

drivers/input/touchscreen/sun4i-ts.c

···
  * in the kernel). So this driver offers straight forward, reliable single
  * touch functionality only.
  *
- * s.a. A20 User Manual "1.15 TP" (Documentation/arm/sunxi.rst)
+ * s.a. A20 User Manual "1.15 TP" (Documentation/arch/arm/sunxi.rst)
  * (looks like the description in the A20 User Manual v1.3 is better
  * than the one in the A10 User Manual v.1.5)
  */

drivers/pwm/pwm-atmel.c

···
  *		 Bo Shen <[e-mail]>
  *
  * Links to reference manuals for the supported PWM chips can be found in
- * Documentation/arm/microchip.rst.
+ * Documentation/arch/arm/microchip.rst.
  *
  * Limitations:
  * - Periods start with the inactive level.

drivers/pwm/pwm-pxa.c

···
  *		eric miao <[e-mail]>
  *
  * Links to reference manuals for some of the supported PWM chips can be found
- * in Documentation/arm/marvell.rst.
+ * in Documentation/arch/arm/marvell.rst.
  *
  * Limitations:
  * - When PWM is stopped, the current PWM period stops abruptly at the next

drivers/tty/serial/Kconfig

···
 	help
 	  If you have a machine based on a SA1100/SA1110 StrongARM(R) CPU you
 	  can enable its onboard serial port by enabling this option.
-	  Please read <file:Documentation/arm/sa1100/serial_uart.rst> for further
-	  info.
+	  Please read <file:Documentation/arch/arm/sa1100/serial_uart.rst> for
+	  further info.
 
 config SERIAL_SA1100_CONSOLE
 	bool "Console on SA1100 serial port"