drivers: spi: Fix spi-gpio to correctly implement sck-idle-input

Formerly, if configured using DT, CS GPIOs were driven from spi.c
and it was possible for CS to be asserted (low) *before* starting
to drive SCK. CS GPIOs have been brought under control of this
driver in both ACPI and DT cases, with a fixup for GPIO polarity.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>