p990: use fs_mgr to mount all the device partitions

Instead of mounting all the device partitions manually from the .rc,
use fs_mgr along with a device specific fstab.

Change-Id: I000c320e4e8ff8dc219860d2357c2b613ca90f13
Signed-off-by: Varun Wadekar <varun.wadekar@gmail.com>