drm/nouveau/clk/gm20b: add glitchless and DFS support

This patch adds support for advanced features supported by the
Noise-Aware PLL of Maxwell. Glitchless switch allows the PL field to be
updated without disabling the PLL first if the SYNC_MODE bit of the CFG
register is set.

More significantly, DFS allows the PLL to monitor the actual input
voltage and to dynamically lower the output frequency accordingly. This
allows the clock to be more tolerant of lower voltages.

These improvements are only supported for Tegra speedos >= 1.

Also add the voltage table that is suitable for GM20B's NAPLL. This
change needs to be done atomically for the right voltages to be used by
the clock driver.

v2. Fix build on non-Tegra platforms

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>