ARM: OMAP2: Switch to use DEVICE_ATTR_RO()

Move device attributes to DEVICE_ATTR_RO() as that would make things
a lot more "obvious" what is happening over the existing __ATTR usage.

Link: https://lore.kernel.org/r/20200523170859.50003-8-sudeep.holla@arm.com


Cc: Tony Lindgren <tony@atomide.com>
Cc: linux-omap@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>