pmdomain: sunxi: sun20i-ppu: change to tristate and enable for ARCH_SUNXI

There is no reason why the sun20i-ppu cannot be built as a module. So
change it to tristate.

Also enable it by default for ARCH_SUNXI since this driver is required
for some peripherals to work, and update the help text to reflect this
requirement.

This aligns it with the new PCK-600 driver.

Signed-off-by: Chen-Yu Tsai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/pmdomain/sunxi/Kconfig

···
 # SPDX-License-Identifier: GPL-2.0-only
 
 config SUN20I_PPU
-	bool "Allwinner D1 PPU power domain driver"
+	tristate "Allwinner D1 PPU power domain driver"
 	depends on ARCH_SUNXI || COMPILE_TEST
 	depends on PM
+	default ARCH_SUNXI
 	select PM_GENERIC_DOMAINS
 	help
-	  Say y to enable the PPU power domain driver. This saves power
-	  when certain peripherals, such as the video engine, are idle.
+	  Say y to enable the PPU power domain driver. This is required
+	  to enable power to certain peripherals, such as the display
+	  engine.
 
 config SUN50I_H6_PRCM_PPU
 	tristate "Allwinner H6 PRCM power domain driver"