rockchip: video: rk_hdmi: fix implicit definition warnings

When enabling CONFIG_DISPLAY_ROCKCHIP_HDMI, compile-time warning for
the following implicitly defined functions are raised due to a missing
include directive:

  drivers/video/rockchip/rk_hdmi.c: In function 'rk_hdmi_probe':
  drivers/video/rockchip/rk_hdmi.c:150:2: warning: implicit declaration of function 'rk_setreg' [-Wimplicit-function-declaration]
    rk_setreg(&priv->grf->soc_con6, 1 << 15);
    ^~~~~~~~~
  drivers/video/rockchip/rk_hdmi.c:153:2: warning: implicit declaration of function 'rk_clrsetreg' [-Wimplicit-function-declaration]
    rk_clrsetreg(&priv->grf->soc_con6, 1 << 4,
    ^~~~~~~~~~~~

This change fixes this by including <asm/hardware.h> in rk_hdmi.c.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>