85xx, 86xx: Determine I2C clock frequencies and store in global_data

Update global_data to define i2c1_clk and i2c2_clk to 85xx and 86xx.

Update the get_clocks() function in 85xx and 86xx to determine the I2C
clock frequency and store it in gd->i2c1_clk and gd->i2c2_clk.

Signed-off-by: Timur Tabi <timur@freescale.com>