clocksource/drivers/ixp4xx: Drop boardfile probe path

The boardfiles for IXP4xx have been deleted. Delete all the
quirks and code dealing with that boot path and rely solely on
device tree boot.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220406205505.2332821-1-linus.walleij@linaro.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>