stm32mp: psci: Implement PSCI system suspend and DRAM SSR

Implement PSCI system suspend and placement of DRAM into SSR while the
CPUs are in suspend. This saves non-trivial amount of power in suspend,
on 2x W632GU6NB-15 ~710mW.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>