MPC85xx: TQM8548: add support for the TQM8548_AG module

The TQM8548_AG is a variant of the TQM8548 module with 1 GiB memory,
CAN and without PCI/PCI-X and RTC. U-Boot can be built for this module
with "$ make TQM8548_AG_config".

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>