drivers: fdc2x1x: Loosen newlib dependency

The FDC2X1X driver depends on newlib in Kconfig. This prevents the
driver from being built in a native_posix testing environment, which uses
an external libc from the host. Allow the driver to be built with an
external libc as well.

Signed-off-by: Tristan Honscheid <honscheid@google.com>