mfd: ti_am335x_tscadc: store physical address

store the physical address of the device in its priv to use it
for DMA addressing in the client drivers.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-for-MFD-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>