doc: efi: Add the EFI-loader API documentation

Bring in the documentation from the efi_loader.h header file, so we can
see the API defined there.

Fix efi_alloc() to avoid a warning.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/api/efi.rst

@@ -54,6 +54,12 @@ drivers, handles, loaded images, and the memory map).
 .. kernel-doc:: cmd/efidebug.c
    :internal:
 
+Overall API
+-----------
+
+.. kernel-doc:: include/efi_loader.h
+   :internal:
+
 Initialization of the UEFI sub-system
 -------------------------------------