drm/i915/cnl: Add registers related to voltage swing sequences.

drm/i915/cnl: Add registers related to voltage swing sequences.

This are the registers and bits needed for the voltage swing
sequence on Cannonlake.

v2: Remove CL_DW5 that was wrongly defined.
v3: Use (1 << 1) instead of (1<<1) as Paulo suggested
    Change DW2 swing sel upper and lower macros to do the
    bit selection instead of definint a table that doesn't
    match the spec. It is based on a Manasi version of it.
    Credits-to: Manasi.
v4: Let SCALING_MODE_SEL flexible. (Manasi)

Cc: Paulo Zanoni <[email protected]>
Cc: Manasi Navare <[email protected]>
Signed-off-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Manasi Navare <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]