MIPS: Alchemy: XXS1500 PCMCIA driver rewrite

Rewritten XXS1500 PCMCIA socket driver, standalone (doesn't depend on
au1000_generic.c) and added carddetect IRQ support.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Linux-PCMCIA <linux-pcmcia@lists.infradead.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>