xen/arm: segregate and split GIC low level functionality

xen/arm: segregate and split GIC low level functionality

GIC driver contains both generic and hardware specific low
level functionality in gic.c file.

With this patch, low level functionality is moved to separate
file gic-v2.c and generic code is kept in gic.c file

Callbacks are registered by low level driver with generic driver
and are called wherever required.

The locking mechanism is not changed.

This helps to separate generic and hardware functionality
and implement future hardware version drivers.

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Tested-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>