clk: tegra: pll: Change misc_reg count from 3 to 6

New SoC's may have more than 3 MISC registers, so bump up the array size
and use a #define to be more informative about the value.

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