drivers: pinctrl_emsdp: add dummy mux for unmuxed peripheral

ARC EMSDP board has some peripherals are internal connected,
such as DW spi1 and DFSS i2c0. They are unmuxed and have fix
connection to spi-flash or sensor. For these peripheral, add
dummy mux type to avoid pinctrl ENOENT error.

Signed-off-by: Siyuan Cheng <siyuanc@synopsys.com>