spi: cadence-qspi: Make reset control optional

In the TI am65 device tree files there is no reset defined. Also
the Linux kernel driver uses devm_reset_control_get_optional_exclusive(..)
to get the reset.

Lets do the same as the kernel does and make thr reset optinal.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>