dev: Use the new ByteOrder param type in SimpleUart

dev: Use the new ByteOrder param type in SimpleUart

Use the new ByteOrder param type in SimpleUart. The default value is
currently little endian. However, it is expected that most users of
this device will use single-byte accesses which aren't affected by
endianness.

Change-Id: I3f5d4ea566e5127474cff976332bd53c5b49b9e2
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33295
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>