arm: dts: lpc32xx: add external memory controller device node

The change adds a description of ARM PrimeCell PL175 memory
controller, which is found on NXP LPC32xx SoCs.

The controller supports up to 4 static memory devices mapped to
0xE000 0000 - 0xE3FF FFFF physical memory area.

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