clk: tegra: pll: Add support for PLLMB for Tegra210

Tegra210 SoC's have 2 PLLs for memory usage. Add plumbing to register
and handle PLLMB.

PLLMB is used to allow switching between 2 PLLM's without having to use
and intermediate backup PLL, as we need to lock the PLL before we can
switch to it.

Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>