This patch addresses the performance problem reported
by Christophe Chanavat (a single call to fdelay_config_pulse
takes 2.1ms to complete).

The time is spent in driving the GPIO enable line that
controls the output stage decoupling. This is done via SPI,
and each spi_xfer takes 420us, which add up to the reported
delay.

The fix leaves the enable pins always on, so the output relay
is not used to control whether the outputs are enabled/disabled;
it is the DCR enable bit who takes this role instead.

This results in an average 8us time for output delay configuration,
3us for output disable.

Conflicts:
	kernel/fd-zio.c