dev: Separate generateDeviceTree into a RiscvUart8250 SimObject.

The only difference between the RiscvUart8250 and the regular Uart8250
is that the Riscv version knows how to generate a device tree node
appropriate for use in a Riscv system. This lets us drop the TARGET_ISA
check from that method, since that should be called iff the target
system is Riscv.

Also update the HiFive platform to use the RiscvUart8250 so that it can
continue to generate device trees successfully.

Change-Id: I306596efffed5e5eed337d3db492d2782ebfaa8d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52144
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed