soc: fsl/qe: Use of_get_child_by_name helper

Use the of_get_child_by_name() helper instead of open coding searching
for the 'firmware' child node. This removes directly accessing the name
pointer as well.

Cc: Qiang Zhao <qiang.zhao@nxp.com>
Cc: Li Yang <leoyang.li@nxp.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Qiang Zhao <qiang.zhao@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>