misc: sram: move reserved block logic out of probe function

No functional change, but now previously overloaded sram_probe() is
greatly simplified and perceptible, reserved regions logic also has
its own space.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>