efi_loader: add missing EFI_RESET_PLATFORM_SPECIFIC

EFI_RESET_PLATFORM_SPECIFIC is one of the values that can be used for the
EFI service ResetSystem. The missing definition is added. The value has to
handled in efi_reset_system().

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