ARM: vexpress: Add fixed regulator for SMSC

SMSC driver requires "vdd33a" and "vddvario" regulator supplies now.

Add fixed regulator describing 3V3 power line (in both motherboard's
Device Trees and the non-DT code) and force fixed regulator config
option if regulators framework is enabled.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>