platforms/mps2: Enable Ethernet for MPS2 Cortex-M4

This commit enables support for Ethernet on MPS2 Cortex-M4. We
are reusing the LAN91 driver and this commit just updates the
peripheral base address per the memory map.

Tested with CTP server and all tests pass.

Signed-off-by: Bilal Wasim <muhammadbilal.wasim@arm.com>