Input: add driver for S1 button of rb532

Mikrotik's Routerboard 532 has two builtin buttons, from which one
triggers a hardware reset. The other one is accessible through GPIO
pin 1. Sadly, this pin is being multiplexed with UART0 input, so
enabling it as interrupt source (as implied by the gpio-keys driver)
is not possible unless UART0 has been turned off. The later one though
is a rather bad idea as the Routerboard is an embedded device with
only a single serial port, so it's almost always used as serial
console device.

This patch adds a driver based on INPUT_POLLDEV, which disables the
UART and reconfigures GPIO pin 1 temporarily while reading the button
state.  This procedure works fine and has been tested as part of
another, unpublished driver for this device.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>