west.yml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  6c8c76fc37f10d7f3ce44474dc2a450f61ff46e0

Brings following Zephyr relevant fixes:
 - 4da51013 zephyr: Add shared data support
 - 9bef51ce bootutil/crypto: Do not include import key with PSA
 - db2024eb espressif: update secure boot and flash encryption

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>