ARM: msm: Allow 8960 and 8660 to compile together

Modify the Kconfig to allow the MSM 8660 and MSM 8960 targets to
compile together in the same build. As long as one of these two
targets is selected the choice menu used to select which SoC to
support will be hidden. Deselecting both of these targets will
reintroduce the choice menu, and allow users to select targets
that must be compiled in isolation.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>