efi/libstub: Describe memory functions

Provide descriptions of:

* efi_get_memory_map()
* efi_low_alloc_above()
* efi_free()

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200218063038.3436-1-xypron.glpk@gmx.de


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>