mtd, nand: Move common functions from cmd_nand.c to common place

mtd, nand: Move common functions from cmd_nand.c to common place

Move common functions from cmd_nand.c (for calculating offset
and size from cmdline paramter) to common place, so they could
used from other commands which use mtd partitions.

For onenand the arg_off_size() is left in common/cmd_onenand.c.
It should use now the common arg_off() function, but as I could
not test onenand I let it there ...

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: Jagannadh Teki <jteki@openedev.com>