CHROMIUM: mfd: chromeos_ec: use threaded irq for EC

CHROMIUM: mfd: chromeos_ec: use threaded irq for EC

It suits better the I2C-based MFD model and will let us more room to
implement IRQ muxing with an irqchip and level triggered interrupt.

The irq thread is scheduled by SCHED_FIFO which is higher priority than
SCHED_OTHER used by the current workqueue.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:9594
TEST=on Daisy2 board, test keyboard input in ChromeOS browser.

(cherry picked from commit c370a8a7d19451adac19cb26bcef06620e2f3d3f)

Change-Id: Ieb8af462d89d792b901381d32f13c9ae28f5e48d
Reviewed-on: https://gerrit-int.chromium.org/17854
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@google.com>
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>