ENGR00281611 i2c: imx: update i2c clock divider for each transaction

ENGR00281611 i2c: imx: update i2c clock divider for each transaction

i.MX6 I2C clk is from IPG_PERCLK which is sourced from IPG_CLK.
Under normal operation, IPG_CLK is 66MHz, ipg_perclk is at 22MHz.
In low bus freq mode, IPG_CLK is at 12MHz and IPG_PERCLK is down
to 4MHz. So the I2C driver must update the divider register for
each transaction when the current IPG_PERCLK is not equal to the
clock of previous transaction.

Signed-off-by: Fugang Duan  <B38611@freescale.com>