omap3: emif4: More clearly hard-code cs0 size

We have a single platform that is both in the OMAP3 family of parts, but
has an EMIF4 memory controller.  Currently we hard-code the size of
chip select 0.  Make this more clear by putting the value in the
function rather than a CONFIG option.

Signed-off-by: Tom Rini <trini@konsulko.com>