docs: marvell: Update build documentation to reflect mrvl_bootimage and mrvl_flash changes

Also add example how to build TF-A for A3720 Turris MOX board and also fix
style/indentation issues and information about default values.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I2dc957307b1b627b403a8d960e85f5ac9e15aee5
1 file changed