serial: lpc32xx: allow compile testing

The lpc32xx_loopback_set() function in hte lpc32xx_hs driver is the
one thing that relies on platform header files. Move that into the
core platform code so we only need a variable declaration for it,
and enable COMPILE_TEST building.

Link: https://lore.kernel.org/r/20190809144043.476786-12-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>