reset: Add get/assert/deassert/release for bulk of reset signals

This patch adds a "bulk" API to the reset API in order to get/deassert/
assert/release a group of reset signals associated with a device.

This bulk API will avoid adding a copy of the same code to manage
a group of reset signals in drivers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>