omap4: Fix multiboot with CONFIG_PM and CONFIG_ARCH_OMAP3 selected

We cannot use the omap34xx_sram_init(). It needs to be implemented
for omap4. Otherwise we get an error and the system won't boot:

Unhandled fault: imprecise external abort (0x1406) at 0xea963e94
...

Fix this by adding a dummy omap44xx_sram_init().

Signed-off-by: Tony Lindgren <tony@atomide.com>