android: persistent_ram: move persistent_ram.h back to include/linux

When persistent_ram went in to staging, persistent_ram.h was moved
from include/linux to driver/staging/android.  To avoid having to
fix all the users, move it back to include/linux.

Signed-off-by: Colin Cross <ccross@android.com>

Change-Id: Ifa73781b4af5412042114a4b9b0476f6c4abb99e