198MHz is a valid operating point for mx6sx.

Add entries for VDD_ARM_CAP and VDD_SOC_CAP voltages for 198MHz according
to the imx6sx datahseet:
http://cache.nxp.com/files/32bit/doc/data_sheet/IMX6SXIEC.pdf

(a 25mV offset is added to the minimum allowed values for safety).

These values also match the ones from the NXP kernel.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>