hx4700: regulator: declare full constraints

Since the removal of CONFIG_REGULATOR_DUMMY option, the touchscreen stopped
working. This patch enables the "replacement" for REGULATOR_DUMMY and
allows the touchscreen to work even though there is no regulator for "vcc".

Cc: stable@vger.kernel.org
Signed-off-by: Martin Vajnar <martin.vajnar@gmail.com>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>