microblaze: Fix EMAC Lite initialization

It is possible for CONFIG_XILINX_EMACLITE to be defined without
XILINX_EMACLITE_BASEADDR being defined as the EMAC Lite driver support
OF init. Check that the driver is enabled and the base address is
available before initializing with a static base address.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>