ASoC: rockchip: pdm: select CONFIG_RATIONAL

Without this, we get a link error:

sound/soc/rockchip/rockchip_pdm.o: In function `rockchip_pdm_hw_params':
rockchip_pdm.c:(.text+0x754): undefined reference to `rational_best_approximation'

Fixes: 624e8e00 ("ASoC: rockchip: pdm: fixup pdm fractional div")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>