dm646x: add new configuration for dm6467T

add new configuration file for dm6467T and appropraite changes
in boards.cfg. dm6467T is the new varaiant of dm6467 SOC which
supports 33 MHz reference clock where as dm6467 supports 27 MHz
reference clock.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>