powerpc/p1023rds: Fix the error of bank-width of nor flash

In the p1023rds, a physical bus of nor flash is 16 bits width.
The bank-width is width (in bytes) of the bus width. So, the
value of bank-width of nor flash is not one, and it should be
two.

Signed-off-by: Chunhe Lan <Chunhe.Lan@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>