powerpc/pmu: Fix ADB_PMU_LED_IDE dependencies

for quite some time the following is printed (twice) after doing
"make oldconfig":

[...]
scripts/kconfig/conf --oldconfig Kconfig
warning: (ADB_PMU_LED_IDE) selects LEDS_TRIGGER_IDE_DISK which has unmet direct dependencies (NEW_LEDS && IDE_GD_ATA && LEDS_TRIGGERS)
warning: (ADB_PMU_LED_IDE) selects LEDS_TRIGGER_IDE_DISK which has unmet direct dependencies (NEW_LEDS && IDE_GD_ATA && LEDS_TRIGGERS)

The following patch causes ADB_PMU_LED to depend on IDE_GD_ATA, so that
the options above are only available when IDE_GD_ATA is actually selected
and thus eliminates the warning.

Signed-off-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>