rtc: stm32: Constify static 'pinctrl_desc'

The local static 'struct pinctrl_desc' is not modified, so can be made
const for code safety.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Alexandre Belloni <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/rtc/rtc-stm32.c

···
 	.strict			= true,
 };
 
-static struct pinctrl_desc stm32_rtc_pdesc = {
+static const struct pinctrl_desc stm32_rtc_pdesc = {
 	.name = DRIVER_NAME,
 	.pins = stm32_rtc_pinctrl_pins,
 	.npins = ARRAY_SIZE(stm32_rtc_pinctrl_pins),