irqchip/gic-v3-its: Probe ITS page size for all GITS_BASERn registers

The GICv3 ITS driver assumes that once it has latched on a page size for
a given BASER register, it can use the same page size as the maximum
page size for all subsequent BASER registers.

Although it worked so far, nothing in the architecture guarantees this,
and Nianyao Tang hit this problem on some undisclosed implementation.

Let's bite the bullet and probe the the supported page size on all BASER
registers before starting to populate the tables. This simplifies the
setup a bit, at the expense of a few additional MMIO accesses.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reported-by: Nianyao Tang <tangnianyao@huawei.com>
Tested-by: Nianyao Tang <tangnianyao@huawei.com>
Link: https://lore.kernel.org/r/1584089195-63897-1-git-send-email-zhangshaokun@hisilicon.com