Include linux/io.h for readl()/writel().

When built on x86_64 w/ COMPILE_TEST=y:
../drivers/gpu/drm/hisilicon/kirin/dw_dsi_reg.h:93:16: error: implicit declaration of function ‘readl’ [-Werror=implicit-function-declaration]
   93 |         orig = readl(addr);
      |                ^~~~~
../drivers/gpu/drm/hisilicon/kirin/dw_dsi_reg.h:96:9: error: implicit declaration of function ‘writel’ [-Werror=implicit-function-declaration]
   96 |         writel(tmp, addr);
      |         ^~~~~~

Cc: Xinliang Liu <xinliang.liu@linaro.org>
Cc: Tian Tao <tiantao6@hisilicon.com>
Cc: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Yongqin Liu <yongqin.liu@linaro.org>
Cc: John Stultz <jstultz@google.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170426.9285-9-ville.syrjala@linux.intel.comAcked-by: John Stultz <jstultz@google.com>