mpc83xx: Change PVR_83xx to PVR_E300C1-3, and update checkcpu().

Rather than misleadingly define PVR_83xx as the specific type of 83xx
being built for, the PVR of each core revision is defined. checkcpu() now
prints the core that it detects, rather than aborting if it doesn't find
what it thinks it wants.

Signed-off-by: Scott Wood <scottwood@freescale.com>