drm/i915/icl: Define AUX lane registers for Port A/B

This patch defines AUX lane registers for PORT_PCS_DW1,
PORT_TX_DW2, PORT_TX_DW4, PORT_TX_DW5 used during
dsi enabling.

v2: Review comments from Jani N:
    - Define _ICL_PORT_PCS_DW1_AUX_A for consistency
    - Three spaces for bitfield definition.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1530798591-2077-8-git-send-email-madhav.chauhan@intel.com