mtd: mtdram: Add parameter for setting writebuf size

ubifs uses the write buffer size in recovery algorithm. When inspecting
an unclean ubifs recovery fails with writebuf size 64 in mtdram while
recovery on actual mtd device with writebuf size of 1024 succeeds.
So add a parameter for setting this property.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>