nand: extend nand torture

nand: extend nand torture

nand torture currently works on exactly one nand block which is specified
by giving the byteoffset to the beginning of the block.

Extend this by allowing for a second parameter specifying the byte size
to be tested.

e.g.
==> nand torture 1000000

NAND torture: device 0 offset 0x1000000 size 0x20000 (block size 0x20000)
 Passed: 1, failed: 0

==> nand torture 1000000 40000

NAND torture: device 0 offset 0x1000000 size 0x40000 (block size 0x20000)
 Passed: 2, failed: 0

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>
[scottwood: fix usage to show size as optional, and add misssing braces]
Signed-off-by: Scott Wood <oss@buserror.net>