cva6: add missing chardev include for dev_defn

[midnightveil]

This was previously giving a build warning regarding it only being defined within this file itself.

/util_libs/libplatsupport/src/mach/cva6/serial.c:12:28: warning: ‘struct dev_defn’ declared inside parameter list will not be visible outside of this definition or declaration
   12 | int uart_init(const struct dev_defn *defn,
      |                            ^~~~~~~~

[lsf37]

Still need to wrap the commit message body at 72 to pass the Gitlint check, but otherwise looks fine to me.