ARM: tegra: pinmux: add note re: drive group field defines

Tegra's drive group registers have a remarkably inconsistent layout. The
current U-Boot driver doesn't take this into account at all. Add a
comment to describe the issue, so at least anyone debugging the driver
will be aware of this. To solve this, we'd need to add a per-drive-group
data structure describing the layout for the individual register. Since
we don't set up too many drive groups in U-Boot at present, this
hopefully isn't causing too much practical issue. Still, we probably need
to fix this sometime.

Wth Tegra210, the register layout becomes almost entirely consistent, so
this problem partially solves itself over time.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>