Tegra: Split tegra_get_chip_type() into soc & sku funcs

As suggested by Stephen Warren, use tegra_get_chip() to return
the pure CHIPID for a Tegra SoC (i.e. 0x20 for Tegra20, 0x30 for
Tegra30, etc.) and rename tegra_get_chip_type() to reflect its true
function, i.e. tegra_get_chip_sku(), which returns an ID like
TEGRA_SOC_T25, TEGRA_SOC_T33, etc.

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