efi/libstub: Describe missing 'out' parameter in efi_load_initrd

The function efi_load_initrd() had a documentation warning due to
the missing description for the 'out' parameter. Add the parameter
description to the kernel-doc comment to resolve the warning and
improve API documentation.

Fixes the following compiler warning:
drivers/firmware/efi/libstub/efi-stub-helper.c:611: warning: Function parameter or struct member 'out' not described in 'efi_load_initrd'

Fixes: f4dc7fff ("efi: libstub: unify initrd loading between architectures")
Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>