ppc4xx: Rework 440GX UIC handling

ppc4xx: Rework 440GX UIC handling

This patch reworks the 440GX interrupt handling so that the common 4xx
code can be used. The 440GX is an exception to all other 4xx variants
by having the cascading interrupt vectors not on UIC0 but on a special
UIC named UICB0 (UIC Base 0). With this patch now, U-Boot references
the 440GX UICB0 when UIC0 is selected. And the common 4xx interrupt
handling is simpler without any 440GX special cases.

Also some additional cleanup to cpu/ppc4xx/interrupt.c is done.

Signed-off-by: Stefan Roese <sr@denx.de>