ARM: davinci: da8xx: Add full regulator constraints for non-DT boot

The phy framework requests an optional "phy" regulator. If it does
not find one, it returns -EPROBE_DEFER. In the case of non-DT boot
for the omap138-lcdk board, this would prevent the usb11 phy to probe
correctly and ohci would not enumerate.

By calling regulator_has_full_constraints(), An error would be returned
instead of -EPROBE_DEFER for the regulator, and the probe of the phy driver
can continue normally without a regulator.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
[nsekhar@ti.com: minor commit message updates]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>