Include the EFI Runtime Code in the DMAP

[andrew]

Summary

Some UEFI implementations place the system table in a runtime code
memory region. Include it in the DMAP so we can read it later.

[kevans]

The way U-Boot does this feels a little sloppy, but nothing in the spec seems to forbid it and the alternative is probably way messier...

The relevant part being here: https://elixir.bootlin.com/u-boot/latest/source/lib/efi_loader/efi_memory.c#L836 -- they end up grouping efi runtime code/data together to minimize padding:

.efi_runtime : {                                                               
               __efi_runtime_start = .;                                        
 *(.text.efi_runtime*)                                                         
 *(.rodata.efi_runtime*)                                                       
 *(.data.efi_runtime*)                                                         
               __efi_runtime_stop = .;                                         
}