[ARM] pxa: add vcc_core regulation for cpufreq on pxa2xx

Add voltage regulation capability to pxa2xx cpufreq
driver. The cpufreq will ask for a "vcc_core" regulator to
the regulator framework.

If a regulator is found at probe time, it will be used with
values specified in PXA270 Electrical, Mechanical, and
Thermal Specifications.

If not, it will be assumed for now that frequency change
will work without voltage control. This assumes that the
IPL/SPL installs sane values to an existing voltage
regulator (ie. voltage high enough to support the full
range).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>