LF-4271-5: arm64: dts: imx8mp: separate dsp_reserved_heap memory

Separate dsp_reserved_heap memory from dsp_reserved for
dsp memory allocation.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Peng Zhang <peng.zhang_8@nxp.com>