NFC: trf7970a: add device tree option for 27MHz clock

The TRF7970A has configuration options to support hardware designs
which use a 27.12MHz clock. This commit adds a device tree option
'clock-frequency' to support configuring the this chip for default
13.56MHz clock or the optional 27.12MHz clock.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Geoff Lansberry <geoff@kuvee.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>