arm/dt: 8974: Tune USB PHY squelch threshold settings

Currently, USB PHY squelch threshold settings (SQRXTUNE) is set to
the power on reset value(0x3). This can cause enumeration failures
if amplitude of the USB signal received is less than the threshold.
Hence, change the SQRXTUNE value from 3 to 6 using PHY's OVERRIDE_B
register.

CRs-Fixed: 386598
Change-Id: I1af10d4c412e021fa6135b8342f93fe608aa7363
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>