xen/arm: Fix PLATFORM_QUIRK_GIC_64K_STRIDE

xen/arm: Fix PLATFORM_QUIRK_GIC_64K_STRIDE

The patch "xen/arm: use ioremap to map gic-v2 registers" handled this
quirk by mapping a 64K+1 page region, but the code continued to
assume that GICC_DIR was at offset 0x1000 from the start of the
mapping.

Fix this by mapping both pages of the GICC space independently and
adjusting the accessor function to use the correct page.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Cc: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Cc: Anup Patel <anup.patel@linaro.org>
Acked-by: Julien Grall <julien.grall@linaro.org>