Input: bcm_iproc_tsc - use syscon to access shared registers

In Cygnus SOC touch screen controller registers are shared with ADC and
flex timer. Using readl/writel could lead to race condition. So touch
screen driver is enhanced to support register access using syscon framework
API's to take care of mutually exclusive access.

Signed-off-by: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>