ARM: dts: msm: Enable U1 and U2 power saving modes for sdx20

Enable U1 and U2 power modes on sdx20. This is needed to also
pass USB CV Ch9 GenX tests in the default configuration.

Change-Id: I2418300230d7d18a59ce50e6b385f9580b657b36
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>