ARM: tegra: tegra2_clocks: 0 -> NULL changes

Fixes a lot of:

arch/arm/mach-tegra/tegra2_clocks.c:921:34: warning: Using plain integer as NULL pointer
arch/arm/mach-tegra/tegra2_clocks.c:1462:4: warning: Using plain integer as NULL pointer
arch/arm/mach-tegra/tegra2_clocks.c:1864:4: warning: Using plain integer as NULL pointer

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>