Add missing 'cdns,max-bit-rate' property for mhdp phy. If not
specified, the default value taken is 8100 Mbps which often leads to
phy link training errors like "CR: max swing reached" and eventually
falls back to 1620 Mbps which limits the maximum supported resolution
to 1920x1200 which is way less than what hardware can actually support
theoretically i.e. 4K.
Adding 2700 (as a HACK) instead of 5400 like other platforms as the
latter causes CRTC SYNC LOST issue for 3840x2160@60fps resolution.
So limiting the phy-rate for max resolution of 3840x2160@30fps as it
is better to have a stable low fps than flaky high fps.
The HACK can be removed when 4K@60fps is stable.

Signed-off-by: Jayesh Choudhary <[email protected]>