microblaze: Add faked LL_TEMAC driver configuration

Expand the specific configuration for the microblaze-generic
board in xparameters.h with a faked setup to enable the
LL_TEMAC driver.

Note: From now the microblaze-generic board is no longer a
valid board configuration for a real piece of hardware. Rather
than, we use the file config.mk and xparameters.h as a faked
board configuration to force the compilation of all potential
driver code for Microblaze systems.

Signed-off-by: Stephan Linz <linz@li-pro.net>