rpi: limit size of the RAM to the multiple of the MMU_SECTION_SIZE

rpi: limit size of the RAM to the multiple of the MMU_SECTION_SIZE

When RPi4 is booted from USB Mass Storage, the firmware reports 947MiB of
the ARM memory (948 in case of the standard SD-card boot). This value is
not MMU_SECTION_SIZE aligned, so the dram_bank_mmu_setup() skips mapping
of the last 1MiB. This later causes u-boot in ARM 32bit mode to freeze,
because it relocated itself into that unmapped memory and fails to
execute.

Fix this by limiting the size of the first bank to the multiple of
MMU_SECTION_SIZE.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I9734e7a3a2b791afff211f57eb872f99bf6f2672