hw/registerfields: Use 64-bit bitfield for FIELD_DP64

hw/registerfields: Use 64-bit bitfield for FIELD_DP64

If we have a field that's wider than 32-bits, we need a data type wide enough to
be able to create the bitfield used to deposit the value.

Signed-off-by: Joe Komlodi <joe.komlodi@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1626805903-162860-3-git-send-email-joe.komlodi@xilinx.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>