efi_loader: Allow boards to implement get_time and reset_system

EFI allows an OS to leverage firmware drivers while the OS is running. In the
generic code we so far had to stub those implementations out, because we would
need board specific knowledge about MMIO setups for it.

However, boards can easily implement those themselves. This patch provides the
framework so that a board can implement its own versions of get_time and
reset_system which would actually do something useful.

While at it we also introduce a simple way for code to reserve MMIO pointers
as runtime available.

Signed-off-by: Alexander Graf <agraf@suse.de>