i2c: mxs: Implement i2c_get/set_bus_speed()

This patch implements the setup and retrieval functions for the I2C
bus speed on the MXS I2C IP.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>