Remove cap-mmc-highspeed property from SDHI2 and SDHI3.

This property is unnecessary as the driver automatically sets
the highspeed capability. Furthermore its use is inconsistent with SDHI0
and SDHI1 which are also highspeed capable but do not have this property
present.

Found by inspection.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>