i2c: mxc: Hide kconfig based control in DM_I2C mode

i2c: mxc: Hide kconfig based control in DM_I2C mode

These options only apply when not using DM_I2C.  When using device
trees, the dt will enable and control the speeds of the I2C
controller(s) and these configuration options have no effect.

So disable them in DM_I2C mode.  Otherwise they show up as decoys, and
make it look like one is enabling I2C controllers and setting the speed
when really it's doing nothing.

However, a system using a SPL build will not use DM_I2C in the SPL, even
if DM_I2C is enabled for the main u-boot.  And so the SPL might use the
kconfig based I2C speed controls while the main u-boot does not.

Cc: Sriram Dash <sriram.dash@nxp.com>
Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Heiko Schocher <hs@denx.de>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>