ARM: dts: lpc32xx: disable MAC controller by default

NXP LPC3220 and LPC3230 SoCs do NOT contain a MAC controller, so,
since for now there is just one dtsi file for all variants of
NXP LPC32xx SoCs, it is reasonable to disable the controller
by default and enable it in device tree files of particular boards.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>