m68knommu: make 5206 UART platform addressing consistent

If we make all UART addressing consistent across all ColdFire family members
then we will be able to remove the duplicated plaform data and use a single
setup for all.

So modify the ColdFire 5206 UART addressing so that:

. UARTs are numbered from 0 up
. base addresses are absolute (not relative to MBAR peripheral register)
. use a common name for IRQs used

Signed-off-by: Greg Ungerer <gerg@uclinux.org>