dt-bindings: sram: describe option to reserve parts of the memory

Some SoCs need parts of their sram for special purposes. So while being part
of the peripheral, it should not be part of the genpool controlling the sram.

Therefore add the option to define reserved regions as subnodes of the
sram-node similar to defining reserved global memory regions.

Originally
Suggested-by: Rob Herring <robherring2@gmail.com>

Using subnodes for reserved regions
Suggested-by: Grant Likely <grant.likely@linaro.org>

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Ulrich Prinz <ulrich.prinz@googlemail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>