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: Ic32aa8ed63bc756cdc10e8d5a5591011a84c4ede
Signed-off-by: Taniya Das <tdas@codeaurora.org>