xshm: geni/geno driver interface.

xshm: geni/geno driver interface.

The Shared Memory driver is using two paired registers called
GENI and GENO in for triggering events/interrupts on modem and Linux host.

It's primary use is notification about read and write indexes changes
on each channel. But GENI/GENO are also used for communicating the
address of the shared memory upon modem boot and to notify about start-up
event such as when IPC channels and CAIF is ready for use.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>