x86/coreboot: Exclude memory regions starting above 4GB

x86/coreboot: Exclude memory regions starting above 4GB

This commit updates the RAM region filtering logic in
board_get_usable_ram_top() to skip any memory regions whose start address
is above 4GB. Previously, only the end address was capped at 4GB, but
regions entirely above this threshold were still considered.

Typically, the following memory map entries would cause
board_get_usable_ram_top() to return 0x100000000, which is incorrect.

    start=00000000, end=00001000, type=16
    start=00001000, end=000a0000, type=1
    start=000a0000, end=000f6000, type=2
    start=000f6000, end=000f7000, type=16
    start=000f7000, end=00100000, type=2
    start=00100000, end=6f170000, type=1
    start=6f170000, end=70000000, type=16
    start=70000000, end=80800000, type=2
    start=e0000000, end=f8000000, type=2
    start=fa000000, end=fc000000, type=2
    start=fc800000, end=fc880000, type=2
    start=fd800000, end=fe800000, type=2
    start=feb00000, end=feb80000, type=2
    start=fec00000, end=fed00000, type=2
    start=fed20000, end=fed80000, type=2
    start=feda1000, end=feda2000, type=2
    start=fedc0000, end=fede0000, type=2
    start=100000000, end=102400000, type=2
    start=102400000, end=47f800000, type=1
    start=4000000000, end=4020000000, type=2

By adding a check to continue the loop if the region's start address
exceeds 0xffffffffULL, the function now properly ignores regions that are
not usable in 32-bit address space.

Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>