iommu/rockchip: Reconstruct to support multi slaves

There are some IPs, such as video encoder/decoder, contains 2 slave iommus,
one for reading and the other for writing. They share the same irq and
clock with master.

This patch reconstructs to support this case by making them share the same
Page Directory, Page Tables and even the register operations.
That means every instruction to the reading MMU registers would be
duplicated to the writing MMU and vice versa.
Signed-off-by: ZhengShunQian <zhengsq@rock-chips.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>