leds: ALIX2: Add dependency to !GPIO_CS5335

The ALIX2 LED driver and the CS5535 GPIO drivers share the same I/O
range which causes a conflict if they're both enabled. Fix this for now
by adding Kconfig dependencies. While at it, also drop the EXPERIMENTAL
flag, as the code has been around for awhile already.

Note that this is a hack. At some point, a real platform support for
this board should be added which handles the LEDs via the leds-gpio
driver.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>