efi_loader: replace UINTN by efi_uintn_t

UINTN is used in the UEFI specification for unsigned integers
matching the bitness of the CPU.

Types in U-Boot should be lower case. The patch replaces it
by efi_uintn_t.

Suggested-by: Simon Glass <sjg@chromium.org>
Suggested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>