defconfig: arm64: add support for msm8994 defconfig

Split out the common msm_defconfig into MSM8994 specific
defconfig. Also update the build-all script to reflect the
changes.

Change-Id: I5df7a0d1f74f843bd2a353a389d42290b8b70590
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>