ARM: tegra: use a CPU freq that all SKUs can support

U-Boot on Tegra30 currently selects a main CPU frequency that cannot be
supported at all on some SKUs, and needs higher VDD_CPU/VDD_CORE values
on some others. This can result in unreliable operation of the main CPUs.

Resolve this by switching to a CPU frequency that can be supported by any
SKU. According to the following link, the maximum supported CPU frequency
of the slowest Tegra30 SKU is 600MHz:

repo http://nv-tegra.nvidia.com/gitweb/?p=linux-2.6.git;a=summary
branch l4t/l4t-r16-r2
path arch/arm/mach-tegra/tegra3_dvfs.c
table cpu_dvfs_table[]

According to that same table, the minimum VDD_CPU required to operate at
that frequency across all SKUs is 1.007V. Given the adjustment resolution
of the TPS65911 PMIC that's used on all Tegra30-based boards we support,
we'll end up using 1.0125V instead.

At that VDD_CPU, tegra3_get_core_floor_mv() in that same file dictates
that VDD_CORE must be at least 1.2V on all SKUs. According to
tegra_core_speedo_mv() (in tegra3_speedo.c in the same source tree),
that voltage is safe for all SKUs.

An alternative would be to port much of the code from tegra3_dvfs.c and
tegra3_speedo.c in the kernel tree mentioned above. That's more work
than I want to take on right now.

While all the currently supported boards use the same regulator chip for
VDD_CPU, different types of regulators are used for VDD_CORE. Hence, we
add some small conditional code to select how VDD_CORE is programmed. If
this becomes more complex in the future as new boards are added, or we
end up adding code to detect the SoC SKU and dynamically determine the
allowed frequency and required voltages, we should probably make this a
runtime call into a function provided by the board file and/or relevant
PMIC driver.

Cc: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
Alban, I don't know which regulator is used for VDD_CORE on Tec-NG. If
could you find this out, I can adjust tec-ng.h too. Thanks.