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Previously a check was done on an ID register at the base of a CPU's
internal USB registers to determine if system interface regsiters were
present.  The check looked for an ID register that had the format
ID[0:5] == ~ID[8:13] as described in the MPC5121 User's Manual to
determine if a MPC5121 or MPC83xx/85xx was being used.

There are two issues with this method:
- The ID register is not defined on the MPC83xx/85xx CPUs, so its
  unclear what is being checked on them.
- Newer CPUs such as the P4080 also don't document the ID register, but
  do share the same format as the MPC5121.  Thus the previous code did
  not set 'have_sysif_regs' properly which results in the P4080 not
  properly initializing its USB ports.

Using the device tree 'compatible' node is a cleaner way to determine if
'have_sysif_regs' should be set and resolves the USB initialization issue
seen on the P4080.

Tested on a P4080-based system and compile tested on mpc512x_defconfig
with Frees...