The following build error is seen with CONFIG_PM=n.

drivers/spi/spi-tegra20-slink.c:1188:12: error:
	'tegra_slink_runtime_suspend' defined but not used
drivers/spi/spi-tegra20-slink.c:1200:12: error:
	'tegra_slink_runtime_resume' defined but not used

Declare the functions only if PM is enabled. While at it, remove the
unnecessary forward declarations.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210907045358.2138282-1-linux@roeck-us.netSigned-off-by: Mark Brown <broonie@kernel.org>