powerpc/mpc5200: Document and tidy irq driver

This patch adds documentation to the mpc5200 interrupt controller
driver and cleans up some minor coding conventions.  It also moves the
contents of mpc52xx_pic.h into the driver proper (except for a small
common bit that is moved to the common mpc52xx.h) because the
information encoded there is not required by any other part of kernel
code.  Finally for code readability sake, the L2_OFFSET shift value
is removed because the code using it resolves to a noop.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>