linux-hikey_git: update kernel branch name
linux-hikey_git: update kernel branch name

Commit 04ec80a78 mentioned in SRCREV_kernel is no longer accessible
from master, and is only present in rpk-v4.9 branch:
  $ git branch --contains 04ec80a78
    rpk-v4.9

Signed-off-by: Andrey Konovalov <6decb425dd3212bf9b96628630c2d12a43fdad79@linaro.org>

fboudra/meta-96boards,kraj/meta-96boards,mrchapp/meta-96boards,96boards/meta-96boards,kraj/meta-96boards