Add minimal viable support for SysMMU v7.x, which can be found in modern
Exynos chips (like Exynos850). SysMMU v7.x may implement VM register
set, and those registers should be initialized properly if present.
Usually 8 translation domains are supported via VM registers (0..7), but
only n=0 (default) is used for now.

Changes are as follows:
  - add enabling the default VID instance before enabling SysMMU
  - use v7 VM register for setting the page table base address
  - use v7 VM register for invalidation

Insights for those changes were taken by comparing the I/O dump
(writel() / readl() operations) for vendor driver and this upstream
driver.

It was tested on E850-96 board, which has SysMMU v7.4 with VM registers
present. The testing was done using "Emulated Translation" registers.
That allows initiating translations with no actual users of that IOMMU,
and checking the result by reading the TLB info from corresponding
registers.

Thanks to Marek, who did let me know it only takes a slight change of
registers to make this driver work with v7.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>