i2c-pxa: prepare clock before use

On OLPC XO-1.75 (MMP2), a WARN_ON() was occurring during boot
since the clock being enabled by i2c-pxa had not been prepared.

Use clk_prepare_enable() to ensure that the prepare operation
has taken place, and use clk_disable_unprepare() in the matching
shutdown paths.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>