vt: add documentation for new boot/sysfs options

Add description to Documentation/kernel-parameters.txt on new options
default_blue, default_grn, default_red, and default_utf8.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>