ARM: s3c64xx: enable sparse IRQ support

This is another prerequisite for enabling multiplatform
support, and it is the part I am least certain about.

I assume it will cause the extra boot message "Cannot
allocate irq_descs @ IRQ%d, assuming pre-allocated" to
be printed, but otherwise work ok. This definitely needs
to be tested on real hardware to see if it works.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>