ram: stm32mp1: Unconditionally enable ASR

Enable DRAM ASR, auto self-refresh, unconditionally. This saves non-trivial
amount of power both at runtime and in suspend (on 2x W632GU6NB-15 ~150mW).

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>