arm: stm32mp: Fix board_get_usable_ram_top() again

Do not access gd->ram_size and assume this is actual valid RAM size. Since commit
777aaaa7 ("common/memsize.c: Fix get_effective_memsize() to check for overflow")
the RAM size may be less than gd->ram_size , call get_effective_memsize() to get
the limited value instead.

The aforementioned commit makes STM32MP15xx boards with 1 GiB of DRAM
at 0xc0000000 hang on boot, which is a grave defect.

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