fs: Fully populate the filesystem method struct

fs: Fully populate the filesystem method struct

There is a structure in fs.c with just a probe method. By adding methods
for other operations, we can avoid lots of #ifdefs and switch()s. As a
first step, create the structure ready for use.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>