msm: cachedump: Drop support for using IMEM

IMEM usage was only either enabled on older targets that we don't support
anymore or via a device tree property (qcom,use-imem-dump-offset) that
wasn't used.  Current and newer targets use TZ to handle this.

Change-Id: I22ada1475b12a66f39abf0525407df8282874536
Signed-off-by: Kumar Gala <galak@codeaurora.org>