I2C: mxc_i2c rework

Rewrite the mxc_i2c driver.
 * This version is much closer to Linux implementation.
 * Fixes IPG_PERCLK being incorrectly used as clock source
 * Fixes behaviour of the driver on iMX51
 * Clean up coding style a bit ;-)

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Jason Hui <jason.hui@linaro.org>
Acked-by: Jason Liu <jason.hui@linro.org>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Jason Liu <jason.hui@linro.org>