ARM: OMAP2+: Move omap_reserve() locally to mach-omap2

omap_reserve() callback is defned only for mach-omap2.
So, moving definition of omap_reserve() to mach-omap2.
This helps is moving plat/omap_secure.h local to
mach-omap2

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>