The switch mask can be overridden by an environment variable
The low 32 bit mask for display 0x24 (AM_1280800P2TZQW)
setenv swm24_a ffffffff

The high 32 bit mask for display 0x00 (TX23D200_18L)
setenv swm00_b ffffffff

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>