memory: stm32-fmc2-ebi: keep power domain on

MP25 FMC2 domain has to be kept on. To handle it throw PSCI OS-initiated,
basic PM for keeping domain on is introduced.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Link: https://lore.kernel.org/r/20240226101428.37791-6-christophe.kerello@foss.st.com


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>