ARM: mxs: Add serial number support for i.MX23, i.MX28 SoCs

i.MX23 and i.MX28 SoCs unique identifiers are factory-programmed
in On-Chip OTP memory. i.MX28's 64-bit unique id is in
HW_OCOTP_OPS2:HW_OCOTP_OPS3 (see MCIMX28 Ref. Man., sec. 20.4.22-23).

i.MX23 provides 32-bit long unique id in HW_OCOTP_OPS3.
Though not clearly documented, there is a clue in sec. 35.9.3.

The unique id is reported in /sys/devices/soc0/serial_number
and in /proc/cpuinfo

Signed-off-by: Ivan Zaentsev <ivan.zaentsev@wirenboard.ru>
Suggested-by: Evgeny Boger <boger@wirenboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>