drivers: pwm_nrfx: Add support for center-aligned mode

Add support for up-and-down counter mode, which aligns the center of
each channel's pulses instead of their initial edges.  This is enabled
on a PWM periphral by adding the "center-aligned" property to the
device tree, e.g.:

    &pwm0 {
        status = "okay";
        center-aligned;
        ch0-pin = <15>;
        ch1-pin = <17>;
        ch1-inverted;
    };

Signed-off-by: Jim Paris <jim@jtan.com>