MIPS: Alchemy: Stop IRQ name sharing

Eliminate the sharing of IRQ names among the differenct Alchemy
variants.  IRQ numbers need no longer be hidden behind a
CONFIG_SOC_AU1XXX symbol: step 1 in my quest to make the Alchemy
code less reliant on a hardcoded subtype.

This patch also renames the GPIO irq number constants. It's really
an interrupt line, NOT a GPIO number!

Code which relied on certain irq numbers to have the same name
across all supported cpu subtypes is changed to determine current
cpu subtype at runtime; in some places this isn't possible so
a "compat" symbol is used.

Run-tested on DB1200.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>