doc/efi: add firmware management protocol to the documentation

The firmware management protocol can be used to manage device firmware.
U-Boot can be configured to provide an implementation.

Document the related functions in the API section.
Signed-off-by: NVincent Stehlé <vincent.stehle@arm.com>
Reviewed-by: NHeinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/api/efi.rst

@@ -166,6 +166,12 @@ Unicode Collation protocol
 .. kernel-doc:: lib/efi_loader/efi_unicode_collation.c
    :internal:
 
+Firmware management protocol
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: lib/efi_loader/efi_firmware.c
+   :internal:
+
 Unit testing
 ------------