drivers: serial: Add xtensa semihosting driver

Add xtensa semihosting driver.

It can't use regular semihosting driver as Xtensa's has it's own
semihosting ABI.

Tested-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>