mmc: jz_mmc: Compile-out write support if disabled

mmc: jz_mmc: Compile-out write support if disabled

Do not build write support, unless it's enabled.

In the SPL case, this change will typically remove
precious bytes (as write support is most often
not needed in SPL).

This is important on this platform, where the maximum
SPL size is 14 KiB.

With gcc v7.3, this change saves 144 bytes producing:

size spl/u-boot-spl
   text	   data	    bss	    dec	    hex	filename
   9240	    752	    712	  10704	   29d0	spl/u-boot-spl

To make the code easier to compile-out and more
readable, a pair of read_data/write_data helpers are created.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>