efi/libstub: Add libstub/mem.c to the documentation tree

Let the description of the efi/libstub/mem.c functions appear in the kernel
API documentation in the following chapters:

    The Linux driver implementer’s API guide
        Linux Firmware API
            UEFI Support
                UEFI stub library functions

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200221035832.144960-1-xypron.glpk@gmx.de
Link: https://lore.kernel.org/r/20200308080859.21568-9-ardb@kernel.org