Add support for Samsung extended AGPS

The AGPS implementation in the GPS chipset used in some Samsung devices
(i9100, i9300) can make use of the Psc field. Adapt the relevant
functions hiding the changes incompatible with other devices under the
AGPS_USE_PSC #define.

Credit to Qaweck from xda-developers forum for finding the meaning
of the field.

Change-Id: Ie4691c79ca379a1f5c0a87500c1b06b56ae7ac0d