OMAP: fix compilation error

Forward-declare platform_device structure in
arch/arm/plat-omap/include/plat/flash.h, otherwise compilation may break
with:

In file included from arch/arm/mach-omap1/flash.c:15:
arch/arm/plat-omap/include/plat/flash.h:14: warning: 'struct platform_device' declared inside parameter list
arch/arm/plat-omap/include/plat/flash.h:14: warning: its scope is only this definition or declaration, which is probably not what you want
arch/arm/mach-omap1/flash.c:16: warning: 'struct platform_device' declared inside parameter list
arch/arm/mach-omap1/flash.c:17: error: conflicting types for 'omap1_set_vpp'
arch/arm/plat-omap/include/plat/flash.h:14: error: previous declaration of 'omap1_set_vpp' was here

Detected and corrected while building for Amstrad Delta, confirmed with
omap1_defconfig.

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