uboot-imx

mmc: don't support write & erase for SPL builds

For SPL builds this is just dead code since we'll only need to read.
Eliminating it results in a significant size reduction for the SPL
binary, which may be critical for certain platforms where the binary
size is highly constrained.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>