Implement voltage switch, supporting modes up to SDR-50.

Based on work by Shinobu Uehara, Rob Taylor, William Towle and Ian Molton.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>