firmware_loader: move firmware/ to drivers/base/firmware_loader/builtin/

Currently, the 'firmware' directory only contains a single Makefile
to embed extra firmware into the kernel.

Move it to the more relevant place.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>