Fix an implicit declaration of usleep_range():

drivers/phy/rockchip/phy-rockchip-dphy-rx0.c: In function 'rk_dphy_enable':
drivers/phy/rockchip/phy-rockchip-dphy-rx0.c:203:2: error: implicit declaration of function 'usleep_range' [-Werror=implicit-function-declaration]

Fixes: 32abcc44 ("media: staging: phy-rockchip-dphy-rx0: add Rockchip MIPI Synopsys DPHY RX0 driver")
Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20200921225618.52529-1-tfiga@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>