MIPS: Loongson64: Remove memory node for builtin-dtb

Builtin DTBS should never contain memory node as memory is
going to be managed by LEFI interface.

Remove memory node to prevent confliction.

Fixes: b1a79260

 ("MIPS: Loongson64: DeviceTree for Loongson-2K1000")
Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>