Calucations of PRAM needs to take into account the 'rootfssize'.
Memory available to the linux kernel 'mem=' is in all cases set to the total
memory size minus the pram size.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Wolfgang Denk <wd@denx.de>