mfd: ti_am335x_tscadc: Use PLATFORM_DEVID_AUTO while registering mfd cells

Use PLATFORM_DEVID_AUTO to number mfd cells while registering, so that
different instances are uniquely identified. This is required in order
to support registering of multiple instances of same ti_am335x_tscadc IP.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>