evita: Increment BOARD_VOLD_MAX_PARTITIONS

Commit CyanogenMod/android_system_vold@d11b833201ae72265d52d606fa084841b44e9a65
reserves the largest partition number for other purposes. Increment MAX
partitions here so internal storage becomes available again.

Change-Id: If0e810d7ba74e16c89fcbed46771eb333621b07c