dt-bindings: misc: tegra-apbmisc: Add missing compatible strings

The compatible string for the Tegra210 APBMISC block was missing from
the bindings. Add it and while at it, rewrite the description of the
compatible string to make it clearer.

Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>