hw/char: Initial commit of Ibex UART

This is the initial commit of the Ibex UART device. Serial TX is
working, while RX has been implemeneted but untested.

This is based on the documentation from:
https://docs.opentitan.org/hw/ip/uart/doc/

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei<zhiwei_liu@c-sky.com>

hw/riscv/Kconfig

@@ -4,6 +4,9 @@ config HTIF
 config HART
     bool
 
+config IBEX
+    bool
+
 config SIFIVE
     bool
     select MSI_NONBROKEN
@@ -29,6 +32,7 @@ config SPIKE
 
 config OPENTITAN
     bool
+    select IBEX
     select HART
     select UNIMP