[POWERPC] Convert powermac ide blink to new led infrastructure

This patch removes the old pmac ide led blink code and
adds generic LED subsystem support for the LED.

It maintains backward compatibility with the old
BLK_DEV_IDE_PMAC_BLINK Kconfig option which now
simply selects the new code and influences the
default trigger.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>