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

Documentation/driver-api/firmware/efi/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+============
+UEFI Support
+============
+
+UEFI stub library functions
+===========================
+
+.. kernel-doc:: drivers/firmware/efi/libstub/mem.c
+   :internal:

Documentation/driver-api/firmware/index.rst

@@ -6,6 +6,7 @@ Linux Firmware API
 
    introduction
    core
+   efi/index
    request_firmware
    other_interfaces