omap: add missing FIQ_START definition required forarch/arm/kernel/fiq.c compilation

Several ARM platforms/machines that use FIQ define their value of FIQ_START.
Since FIQ is not implemented for OMAP yet, this definition is missing from
OMAP header files.

Put an arbitrary value for FIQ_START into plat/irqs.h for OMAP. Even if not
used by the FIQ handler for Amstrad Delta, this is required for successfull
compilation of arch/arm/plat-omap/fiq.c that provides several
usefull functions.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Tony Lindgren <tony@atomide.com>