arm64: dts: ti: k3-j721e-vision-apps: correctly define overlay node

The node "ipc-memories@aa000000" is declared as a sub node of
reserved_memory in the dts file "k3-j721e-som-p0.dtsi", here to overlay
the sub node, it should be put in correct place with sub node name.

To fix below build warning:
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtso:63.2-42: Warning (reg_format):
	/fragment@9/__overlay__:reg: property has invalid length (16 bytes) (#address-cells == 2, #size-cells == 1)
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtbo: Warning (pci_device_bus_num): Failed prerequisite 'reg_format'
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtbo: Warning (i2c_bus_reg): Failed prerequisite 'reg_format'
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtbo: Warning (spi_bus_reg): Failed prerequisite 'reg_format
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtso:62.25-64.3: Warning (avoid_default_addr_size):
	/fragment@9/__overlay__: Relying on default #address-cells value
arch/arm64/boot/dts/ti/k3-j721e-vision-apps.dtso:62.25-64.3: Warning (avoid_default_addr_size):
	/fragment@9/__overlay__: Relying on default #size-cells value

Signed-off-by: Xulin Sun <xulin.sun@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>