Input: add dm355evm_keys driver

Simple input driver support for the events reported by the
MSP430 firmware on the DM355 EVM.  Verified using the RC5
remote included with the kit; docs weren't quite right.

Some of the keycode selections might need improvement; they
can be remapped, so there's at least a runtime workaround.
(I also suspect Linux may someday merit more generic support
for RC5 based remote controls.)

These events don't distinguish key press vs release events,
so this reports both and then skips the next event if it's
identical.  The RC5 remote codes include a "toggle" bit that
can help detect autorepeated keys; but this driver doesn't
bother with those nuances.

This driver relies on the drivers/mfd/dm355evm_msp.c code
for core features, including sharing I2C access to this
firmware with GPIO, LED, and RTC support.

[dtor@mail.ru: fix error unwindng path in probe()]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>