ARM: dts: imx6sx-sdb: Add 198MHz operational point

imx6sx-sdb has custom operating points entries because it has one
power supply that drives both VDDARM_IN and VDDSOC_IN.

As per the MX6UL datasheet we have the following minimum voltages for
198 MHz operation (after adding the 25mV margin value):
VDDARM_IN = 0.975 V
VDDSOC_IN = 1.175 V

So use 1.175V for the 198MHz operation.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>