arm: dts: omap: add reg-shift to serial device tree node

With the commit 'c7b9686d ("ns16550: unify serial_omap")' all
TI platforms are broken with DM/DT boot as ns16550 driver expects
reg-shift from DT which is not populated for TI platforms.
Earlier it worked as it was hard coded to 2 in serial-omap
driver. So adding the reg-shift to serial nodes for dra7, am4372
and am33xx dtsi files. Tested this patch on am437x-sk-evm,
am437x-gp-evm, am335x-boneblack, dra74x-evm and dra72x-evm.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Thomas Chou <thomas@wytron.com.tw>