dts: add reg-shift property to all ns16550 devices

The ns16550 flags reg-shift property as optional. In case it is not
supplied, the ns16550 driver relies on a value defined in <soc.h>, or,
by default it takes 4 (shift by 2).

This patch adds the property to all ns16550 nodes, with the following
values:

- 2 if SoC did not have any custom value defined by
  UART_REG_ADDR_INTERVAL (corresponds to 1 << 2 = 4)
- If SoC defined DEFAULT_REG_INTERVAL (snps_arc_iot/it8xxx2), use such
  value (4=2, 2=1, 1=0).

These changes will allow simplifying the ns16550 driver.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>