ARM: cygnus: fix const declaration bcm_cygnus_dt_compat

The const declaration for char* is actually duplicated, however
the array of strings is currently not constant. However, typically
the dt_compat array is declared as const char *const. Follow
that convention and also add the __initconst macro for constant
initialization data.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Scott Branden <sbranden@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Tested-by: Ray Jui <rjui@broadcom.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>