NFC: Make EN2 pin optional in the TRF7970A driver

Make the EN2 pin optional. This is useful for boards,
which have this pin fix wired, for example to ground.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guan Ben <ben.guan@cn.bosch.com>
Signed-off-by: Mark Jonas <mark.jonas@de.bosch.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>