ARM: tegra: Support Tegra30 in decompressor UART setup

On Tegra20, the UART clock runs at 216MHz, whereas on Tegra30 it runs at
408MHz. Modify arch_decomp_setup() to detect Tegra20-vs-Tegra30 at run-
time, and program the correct divisor.

This makes uncompressor messages work correctly on Tegra30. This also
fixes early printk, assuming zImage is used and this setup code runs.

v2: Use CHIPID register to differentiate between chips, rather than a
GIC register. This should be more future-proof. Volatile is required
to prevent the compiler transforming the 32-bit apb_misc register read
into an 8-bit read of address 1 higher, since the HW only supports 32-
bit accesses, and will hang on an 8-bit access.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>