mx6: Fix get_board_rev() for the mx6 solo case

When booting a Freescale kernel 3.0.35 on a Wandboard solo, the get_board_rev()
returns 0x62xxx, which is not a value understood by the VPU
(Video Processing Unit) library in the kernel and causes the video playback to
fail.

The expected values for get_board_rev are:
0x63xxx: For mx6quad/dual
0x61xxx: For mx6dual-lite/solo

So adjust get_board_rev() accordingly and make it as weak function, so that we
do not need to define it in every mx6 board file.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Eric Nelson <eric.nelson@boundarydevices.com>