dts: bindings: serial: smartbond: Fix baudrates

Serial speeds listed in device tree did not reflect
what driver supports.

2M and 500K were missing while
1200, 2400 and 460800 were present while not supported.

This change synchronized dts with driver code:
drivers/serial/uart_smartbond.c

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>