ARM: OMAP2+: GPMC: resolve type-conversion warning from sparse

arch/arm/mach-omap2/gpmc.c passes a return value from ioremap() as the
fifth argument to request_irq() without casting it.  This causes
sparse to generate the following warning:

arch/arm/mach-omap2/gpmc.c:759:63: warning: incorrect type in argument 5 (different address spaces)
arch/arm/mach-omap2/gpmc.c:759:63:    expected void *dev
arch/arm/mach-omap2/gpmc.c:759:63:    got void [noderef] <asn:2>*static [toplevel] [assigned] gpmc_base

It turns out that it's not necessary to pass this.  gpmc_base is a
file-scoped static variable, the ISR is located in the same file ... and
the ISR doesn't even touch the passed-in variable.  So, just replace it with
NULL in request_irq().

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