sram: remove SCM read from plat-omap/sram.c

This patch removes the System Control Module read from
plat-omap/sram.c:is_sram_locked().  Instead, it uses the global system_reg
variable, which is initialized immediately prior to the SRAM init code.
This happens to fix the compile breakage on the current tree.

In the long term, is_sram_locked() probably should be moved out of
sram.c.

Boot-tested on N800.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>