Not that barebox has a proper NVMEM subsystem, convert OCOTP driver to
use that to both make things more consistent with Linux and also allow
accessing OCOTP fields without the need for
imx_ocotp_read_field()/imx_ocotp_write_field().

Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>