ARM: dts: keystone-k2g: Fix unit-address formatting for serial nodes

The unit addresses on couple of serial nodes have a leading 0 resulting
in couple of warning messages around simple-bus unit address format
errors when compiled with W=1. Fix the same.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>