ARM: cache-l2x0: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change the
following warning would occur:

	arch/arm/mm/cache-l2x0.c: In function 'l2x0_of_init':
	arch/arm/mm/cache-l2x0.c:573:7: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

though.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>