When running U-Boot as an EFI payload with CONFIG_EFI_STUB, the reserved
regions from the previous stage EFI bootloader should be carried over
since they may not fully align with the reserved-memory regions in
devicetree.
Implement a helper to map these pages when the EFI subsystem starts up.
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>