mfd: ti_am335x_tscadc: Add DT support

Add DT support in the MFD core driver. The node name is "am3359" because
it was tested on this platform.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Patil, Rachna <rachna@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>