efi_loader: EFI_SIMPLE_TEXT_INPUT_EX_PROTOCOL

This patch implements the EFI_SIMPLE_TEXT_INPUT_EX_PROTOCOL.

The implementation of notification functions is postponed to a later
patch.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>