Ensure CONFIG_RATIONAL is selected in order to fix the following link
error with some kernel configurations:

drivers/phy/rockchip/phy-rockchip-samsung-hdptx.o: in function `rk_hdptx_ropll_tmds_cmn_config':
phy-rockchip-samsung-hdptx.c:(.text+0x950): undefined reference to `rational_best_approximation'

Fixes: 553be283 ("phy: rockchip: Add Samsung HDMI/eDP Combo PHY driver")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404090540.2l1TEkDF-lkp@intel.com/Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20240408222926.32708-1-cristian.ciocaltea@collabora.comSigned-off-by: Vinod Koul <vkoul@kernel.org>