Add note to UPDATING about UEFI changes requiring loader(8) update

These problems have only been observed with boards using U-Boot (e.g. ARM)
where virtual addresses are already set in the memory map by the firmware
and the firmware is expecting a call to SetVirtualAddressMap to be made.
I refrain from mentioning this in the note because this could also be the
case on some not-yet-tested firmware on amd64 and it's not a bad
recommendation for the general case.