xen/arm: use ioremap to map gic-v2 registers

xen/arm: use ioremap to map gic-v2 registers

gic-v2 driver uses fixmap to map the registers.
Instead use ioremap to access mmio registers.

With this patch, gic-v2 register definitions are updated
to use absolute offset address instead of dividing the
register offset by 4.

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