Adds an equivalent of SOC_ENUM_EXT for value enums

Strictly speaking SOC_ENUM_EXT can also be used to define
a value enum since the only difference is the get and set
functions. But this doesn't look good in code because it is
inconsistent with the normal control definitions. Adding a
specific SOC_VALUE_ENUM_EXT is better for code clarity.
Signed-off-by: NRichard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: NMark Brown <broonie@linaro.org>