ARM: OMAP: Fix SRAM W+X mapping

We are still using custom SRAM code for some SoCs and are not marking
the PM code mapped to SRAM as read-only and executable after we're
done. With CONFIG_DEBUG_WX=y, we will get "Found insecure W+X mapping
at address" warning.

Let's fix this issue the same way as commit 728bbe75 ("misc: sram:
Introduce support code for protect-exec sram type") is doing for
drivers/misc/sram-exec.c.

On omap3, we need to restore SRAM when returning from off mode after
idle, so init time configuration is not enough.

And as we no longer have users for omap_sram_push_address() we can
make it static while at it.

Note that eventually we should be using sram-exec.c for all SoCs.

Cc: stable@vger.kernel.org	# v4.12+
Cc: Dave Gerlach <d-gerlach@ti.com>
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>