ARM: omap2: hide omap3_save_secure_ram on non-OMAP3 builds

In configurations without CONFIG_OMAP3 but with secure RAM support,
we now run into a link failure:

arch/arm/mach-omap2/omap-secure.o: In function `omap3_save_secure_ram':
omap-secure.c:(.text+0x130): undefined reference to `save_secure_ram_context'

The omap3_save_secure_ram() function is only called from the OMAP34xx
power management code, so we can simply hide that function in the
appropriate #ifdef.

Fixes: d09220a8

 ("ARM: OMAP2+: Fix SRAM virt to phys translation for save_secure_ram_context")
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>