USB: add usb-serial spcp8x5 driver

Original version of the driver done by Linxb, changes by Harald, and
lots of cleanups by me in order to get it into a mergable state.

Cc: Linxb <xubin.lin@worldplus.com.cn>
Cc: Harald Klein <hari@vt100.at>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>