ARM: dts: imx6sx-sdb: Remove OPP override

The board file for imx6sx-sdb overrides cpufreq operating points to use
higher voltages. This is done because the board has a shared rail for
VDD_ARM_IN and VDD_SOC_IN and when using LDO bypass the shared voltage
needs to be a value suitable for both ARM and SOC.

This only applies to LDO bypass mode, a feature not present in upstream.
When LDOs are enabled the effect is to use higher voltages than necessary
for no good reason.

Setting these higher voltages can make some boards fail to boot with ugly
semi-random crashes reminiscent of memory corruption. These failures only
happen on board rev. C, rev. B is reported to still work.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Fixes: 54183bd7

 ("ARM: imx6sx-sdb: add revb board and make it default")
Cc: stable@vger.kernel.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>