ppc4xx: Optimize PLB4 Arbiter and Memory Queue settings for PPC440SP/SPe,

ppc4xx: Optimize PLB4 Arbiter and Memory Queue settings for PPC440SP/SPe,
	PPC405EX and PPC460EX/GT/SX

- Read pipeline depth set to 4 for PPC440SP/SPE, PPC405EX, PPC460EX/GT/SX
  processors
- Moved PLB4 Arbiter register definitions to ppc4xx.h since it is shared
  across processors (405 and 440/460)
- Optimize Memory Queue settings for PPC440SP/SPE and PPC460EX/GT/SX
  processors
- Add register bit definitions for Memory Queue Configuration registers

Signed-off-by: Prodyut Hazarika <phazarika@amcc.com>
Signed-off-by: Stefan Roese <sr@denx.de>