common/memsize.c: Check for overflow in get_effective_memsize() only for mpc85xx

This reverts commit 777aaaa7 ("common/memsize.c: Fix
get_effective_memsize() to check for overflow") for non-mpc85xx platforms.

The changes to this generic function, which is intended to help with
32bit platforms with large amounts of memory has unintended side effects
(which in turn lead to boot failures) on other platforms which were
previously functional.

For now do overflow check only for powerpc mpc85xx platform. It is needed
to prevent crashing of P1/P2 boards with 4GB DDR module in 32-bit mode.

Fixes: 777aaaa7 ("common/memsize.c: Fix get_effective_memsize() to check for overflow")
Signed-off-by: Pali Rohár <pali@kernel.org>