ARM: OMAP2+: Drop DISPC L3 firewall code

This is only needed when using SRAM for framebuffer,
and the support for SRAM framebuffer is about to get
removed.

Otherwise we cannot move most of plat/io.h to be a local
iomap.h for mach-omap2.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>