Add support for obtaining from DT the SRAM region to store the jump stub
for CPU core bringup, according to the renesas,smp-sram DT bindings.

If no region is specified in DT, the code falls back to hardcoded ICRAM1
as before, to maintain backwards compatibility.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>