mpc83xx: add support for more system clock performance controls

System registers that are modified are the Arbiter Configuration
Register (ACR), the System Priority Control Register (SPCR), and the
System Clock Configuration Register (SCCR).

Signed-off by: Michael F. Reiss <Michael.F.Reiss@freescale.com>
Signed-off by: Joe D'Abbraccio <ljd015@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>