rtc: use genirq directly in rtc-wm831x

Now that the WM831x core uses genirq for the IRQ controller there is no
need to use the WM831x-specific wrappers to request interrupts so convert
to use genirq directly.

Also use more meaningful strings to make /proc/interrupts more readily
legible.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>