imx21: activate i2c

Set the correct clkdev-name for the i2c clock.

It also get's rid of the ARCH_NR_GPIOS define on the rationale
that isn't an ARCH-wide setting anyway. If a device has two
pca953x devices, the reserved number will be wrong.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>