x86: efi: payload: Count in conventional memory above 4GB in DRAM bank

At present in dram_init_banksize() it ignores conventional memory
above 4GB. This leads to wrong DRAM size is printed during boot.
Remove such limitation.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>