ARM: dts: imx6qdl-sr-som-ti: use fixed-clock instead of clock-frequency

ARM: dts: imx6qdl-sr-som-ti: use fixed-clock instead of clock-frequency

Replace the clock-frequency property with a fixed-clock providing the
32kHz input to the gpio-gate-clock.

Fix the following CHECK_DTBS warning:
  ti-wifi-clock (gpio-gate-clock): 'clock-frequency' does not match any of the regexes: '^pinctrl-[0-9]+$'

Signed-off-by: Frank Li <Frank.Li@nxp.com>