ARM: OMAP: fix 'using plain integer as NULL pointer' sparse warnings

sparse warns when 0 is passed to a function expecting a pointer argument.
Resolve these warnings by replacing the 0 with NULL.

arch/arm/plat-omap/include/plat/dmtimer.h:319:34: warning: Using plain integer as NULL pointer
arch/arm/plat-omap/include/plat/dmtimer.h:324:35: warning: Using plain integer as NULL pointer
arch/arm/mach-omap2/irq.c:294:22: warning: Using plain integer as NULL pointer
arch/arm/mach-omap1/board-palmz71.c:292:50: warning: Using plain integer as NULL pointer
arch/arm/mach-omap1/board-palmz71.c:295:73: warning: Using plain integer as NULL pointer
arch/arm/mach-omap1/ams-delta-fiq.c:105:63: warning: Using plain integer as NULL pointer

Signed-off-by: Paul Walmsley <paul@pwsan.com>