ARM: tegra: call cpu_do_idle from C code

When building a kernel for multiple CPU architecture levels,
cpu_do_idle() is a macro for an indirect function call, which
cannot be called from assembly code as Tegra does.

Adding a trivial C wrapper for this function lets us build
a tegra kernel with ARMv6 support enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Joseph Lo <josephl@nvidia.com>
Cc: Stephen Warren <swarren@nvidia.com>