serial: clps711x: Enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.
To do this, read{write}_relaxed() functions was be replaced with
simple read{write}() variants. Potential "uninitialized variable"
warnings was be fixed if driver compiled without MFD_SYSCON.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>