clk: qcom: define vdd_levels enum and vdd_corner[] map

Define a vdd_levels enum and vdd_corner[] mapping table which can
be used by clock controller drivers to configure the vdd_class
associated with their clocks.

Change-Id: Ia0c2ed35f7c87cc3dc9947844625a463745e2ac5
Signed-off-by: David Collins <collinsd@codeaurora.org>