misc: sram: Split sram data structures into local header

In preparation of a coming file split of the sram driver, move the
common data structures into a local header file that can be shared
between files related to the sram driver.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>