pinctrl: baytrail: Add pinconf group for uart3

GPIO_S0_SC57 / GPIO_S0_SC61 can be muxed to PCU_UART_TXD / PCU_UART_RXD,
add a pinconf group for this.

On Bay Trail board schematics using these pins as UART these are
called UART3_TXD / UART3_RXD, name the pinconf group "uart3_grp"
to be consistent with the schematics.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>