ARM: dts: am33xx: Add pm-sram phandle to soc node

Add a phandle to point to both the pm-sram-code and pm-sram-data nodes
so that the pm code can locate the sram regions needed to copy low level
PM code.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>