ARM: 8804/1: zImage: atags_to_fdt: add serial-number for ATAG_SERIAL

If the system passes an ATAG_SERIAL, convert that into a /serial-number
node so that the system serial number will be passed through the FDT and
be present under the kernel.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>