kernel/uart: Clean up uarts, make calling convention consistent

Change-Id: I36ef92ef13edb0b006344db74e9d1b3ae52e0127
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4327
Reviewed-by: X512 <danger_mail@list.ru>
Reviewed-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

kernel/riscv64: Fix GetChar data val assignment for sifive uart

Change-Id: I81981d23222f81ee936695c3257e8185a7bcebfd

kernel: add SiFive UART driver for riscv64

Change-Id: I37a909b5bcdb18b3fa062961eca9ff2f5187867c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4305
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>