ARM: LPC32xx: DT conversion of Standard UARTs

This patch switches from static serial driver initialization to devicetree
configuration. This way, the Standard UARTs of the LPC32xx SoC can be enabled
individually via DT.

E.g., instead of Kconfig configuration, the phy3250.dts activates
UARTs 3 and 5.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Tested-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>