uboot-sign: support 64bits address

uboot-sign: support 64bits address

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

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>