ramips: add Semtech SX9512 touch input driver
This driver is required for the touch-inputs as well as some LEDs on the
Genexis Pulse EX400.
The driver was also sent upstream. As the EX400 is currently the only
consumer, the driver is added target-specific for ramips.
Once the driver has been accepted upstream and is provided in a kernel
release used by OpenWrt, the package should be moved to the global input
drivers.
Link: https://www.spinics.net/lists/kernel/msg5669349.html
Signed-off-by: David Bauer <mail@david-bauer.net>