powerpc/legacy_serial: Bail if reg-offset/shift properties are present

The legacy serial driver does not work with an 8250 type UART that is
described in the device tree with the reg-offset and reg-shift
properties.  This change makes legacy_serial ignore these devices.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>