defconfig: arm64: Decouple MSM8994 from the common msm_defconfig

Remove MSM8994 from the common msm_defconfig.
Also, sync up msm8994_defconfig to be the minimal defconifg
and clean it up.

Change-Id: Iaa1a9212998cdad664c43792d015c6c359826f4b
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>