ARM: tegra: remove common.c

common.c was create to contain code shared across the various Tegra board
files. There is now only one board file, tegra.c. So, move the code there.
One exception is the PMC reboot routine, which moves to pmc.c, and now
takes advantage of the 'standard' tegra_pmc_readl/writel functions.

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