efi: Add shared FDT related functions for ARM/ARM64

efi: Add shared FDT related functions for ARM/ARM64

Both ARM and ARM64 stubs will update the device tree that they pass to
the kernel.  In both cases they primarily need to add the same UEFI
related information, so the function can be shared.  Create a new FDT
related file for this to avoid use of architecture #ifdefs in
efi-stub-helper.c.

Signed-off-by: Roy Franz <[email protected]>
[ Fixed memory node deletion code. ]
Signed-off-by: Mark Rutland <[email protected]>
Signed-off-by: Leif Lindholm <[email protected]>
Acked-by: Grant Likely <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>