wandboard: Don't use I2C speed Kconfig settings with DM_I2C

wandboard: Don't use I2C speed Kconfig settings with DM_I2C

When using DM_I2C the speed value supplied to setup_i2c() is not used,
so this code required CONFIG_SYS_MXC_I2C[12]_SPEED to be defined to
compile, but did not actually use them.

Change this so we no longer need to define an unused macro to compile in
DM_I2C mode.  Also make it more clear that they do not control the bus
speed.  Otherwise it is quite easy to mistakenly believe they are used
to set the bus speed.

Cc: Heiko Schocher <hs@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>