x86, efi: Make efi_memblock_x86_reserve_range more readable

So basically this function copies EFI memmap stuff from boot_params into
the EFI memmap descriptor and reserves memory for it. Make it much more
readable.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Matthew Garret <mjg59@srcf.ucam.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>