drm/sun4i: hdmi: Use more portable I/O helpers

readsb/writesb are unavailable on some architectures. In preparation for
removing the Kconfig architecture dependency, switch to the equivalent
but more portable ioread/write8_rep helpers.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424162633.12369-5-samuel@sholland.org