dts: nxp: Add sram_l node to k64 dtsi

Adds a second sram node to the k64 device tree to acknowledge that the
additional sram is present in hardware, but deliberately not used in
zephyr until an outstanding issue is solved. The upper and lower sram
nodes are contiguous in the memory map, however the Cortex-M4
architecture does not support misaligned accesses across the boundary
between the two nodes.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>