Remove calling CPU mpidr from bakery lock API

The bakery lock code currently expects the calling code to pass
the MPIDR_EL1 of the current CPU.

This is not always done correctly. Also the change to provide
inline access to system registers makes it more efficient for the
bakery lock code to obtain the MPIDR_EL1 directly.

This change removes the mpidr parameter from the bakery lock
interface, and results in a code reduction of 160 bytes for the
ARM FVP port.

Fixes ARM-software/tf-issues#213

Change-Id: I7ec7bd117bcc9794a0d948990fcf3336a367d543