ARM: dts: stm32: omit unused pinctrl groups from stm32mp15 dtb files

stm32mp15-pinctrl.dtsi contains nearly all pinctrl groups collected from
all boards. Most of them end up unused by a board and only waste binary
space. Add /omit-if-no-ref/ to the groups to scrub the unused groups
from the dtbs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>