Driver for the S1 button of rb532

[Phil Sutter <n0-1@freewrt.org>]

Hi,

I've written a polled input device driver for the S1 button built into
the Routerboard 532. Sadly, the gpio-keys driver can't be used, as it
turns on the GPIO as interrupt source (which then has to stay configured
for this use), which would render the serial console unusable as it uses
the same pin in alternate function mode.

I've split up the changes into two parts to divide BSP changes from the
actual driver.

Greetings, Phil