ARM: SPEAr3xx: shirq: simplify and move the shared irq multiplexor to DT

SPEAr3xx architecture includes shared/multiplexed irqs for certain set
of devices. The multiplexor provides a single interrupt to parent
interrupt controller (VIC) on behalf of a group of devices.

There can be multiple groups available on SPEAr3xx variants but not
exceeding 4. The number of devices in a group can differ, further they
may share same set of status/mask registers spanning across different
bit masks. Also in some cases the group may not have enable or other
registers. This makes software little complex.

Present implementation was non-DT and had few complex data structures to
decipher banks, number of irqs supported, mask and registers involved.

This patch simplifies the overall design and convert it in to DT.  It
also removes all registration from individual SoC files and bring them
in to common shirq.c.

Also updated the corresponding documentation for DT binding of shirq.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>