kernel-fitimage: support 64 bits address

kernel-fitimage: support 64 bits address

The default value of address-cells is "1", so the generated "its" file
only supports 32bits address for kernel FIT image.
However, some platforms may want to support 64bits address
of UBOOT_LOADADDRESS and UBOOT_ENTRYPOINT.
Therefore, adds a variable to support both 64bits and 32bits address cells.

By default, the address-cell is "1" which is used for 32bits load address.
If users would like to use 64bits load address, users are able to
set as following for "0x400000000" 64bits load address.
1. FIT_ADDRESS_CELLS = "2"
2. UBOOT_LOADADDRESS= "0x04 0x00000000"

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>