MIPS: dts: remove bogus bcm96358nb4ser.dtb from dtb-y entry

arch/mips/boot/dts/brcm/bcm96358nb4ser.dts does not exist, so
we cannot build bcm96358nb4ser.dtb .

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: 69583551 ("MIPS: BMIPS: rename bcm96358nb4ser to bcm6358-neufbox4-sercom")
Cc: stable@vger.kernel.org # 4.9+
Acked-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>