ARM: tegra: build powergate unconditionally

powergate functions are needed for tegra30 as well (see common.c), so
build it always. Fixes:

arch/arm/mach-tegra/built-in.o: In function `tegra30_init_early':
apbio.c:(.init.text+0x78): undefined reference to `tegra_powergate_init'

(using "allnoconfig" with tegra30 enabled)

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>