msm: board-8960: Only register ext_l2 regulator on CDPs

The external LDO 2 regulator, ext_l2, is enabled via MSM GPIO 91
on 8960 CDP only.  However, the ext_l2 device is added in the
board file unconditionally for all 8960 variants.  Change this
so that ext_l2 is only registered for CDPs.

Change-Id: I8b6b81503c00499982e8477021c61adb669e5218
Signed-off-by: David Collins <collinsd@codeaurora.org>