soc/tegra: Fix link errors with PMC disabled

With the new Tegra186 PMC driver merged, anything that relies on the previous
PMC driver fails to link when that is disabled:

arch/arm/mach-tegra/pm.o: In function `tegra_pm_set':
pm.c:(.text.tegra_pm_set+0x3c): undefined reference to `tegra_pmc_enter_suspend_mode'
arch/arm/mach-tegra/pm.o: In function `tegra_suspend_enter':
pm.c:(.text.tegra_suspend_enter+0x4): undefined reference to `tegra_pmc_get_suspend_mode'
arch/arm/mach-tegra/pm.o: In function `tegra_init_suspend':
pm.c:(.init.text+0x1c): undefined reference to `tegra_pmc_get_suspend_mode'
pm.c:(.init.text+0x74): undefined reference to `tegra_pmc_set_suspend_mode'

ERROR: tegra_powergate_sequence_power_up [drivers/ata/ahci_tegra.ko] undefined!
ERROR: tegra_powergate_power_off [drivers/ata/ahci_tegra.ko] undefined!

Making the definition depend on the presence of the driver makes it build
again, though that might not be the correct fix.

Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Fixes: 854014236290 ("soc/tegra: Implement Tegra186 PMC support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>