drivers: cpuidle: Remove cpuidle-arm64 duplicate error messages

Current CPUidle driver for arm64 machines spits errors upon idle
state initialization and cpuidle driver registration failures.
These error messages are already printed in core code so there is
no need to print them again.

This patch removes the duplicate print messages from the cpuidle-arm64
driver.

Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>