x86: doc: Fix reference to EFI doc in U-Boot

Since commit f3b5056c4e72 ("efi_loader: split README.efi into two
separate documents"), the original README.efi was renamed to
README.u-boot_on_efi, but x86 doc still refers to the old one.

This updates the x86 doc to reference both README.u-boot_on_efi and
README.uefi.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed