drivers: i2s_nrfx: Generate master clock if pin is connected

The driver uses pinctrl to configure pins instead of nrfx I2S API.
Check whether MCK pin was actually connected by pinctrl instead of
comparing nrfx_cfg.mck_pin that is always NRF_I2S_PIN_NOT_CONNECTED.

This makes it possible for nRF I2S to provide master clock even when
operating in I2S Slave mode.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>