gpu: host1x: Simplify register mapping and add common aperture

Refactor 'regs' property loading using devm_platform_ioremap_*
and add loading of the 'common' region found on Tegra234.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>