OMAP3 Fix compiler warning for v7_flush_dcache_all

On build of omap3 targets in MAKEALL, the *.ERR files have

cpu.c: In function 'cleanup_before_linux':
cpu.c:64: warning: implicit declaration of function 'v7_flush_dcache_all'
cpu.c:64: warning: implicit declaration of function 'get_device_type

The functions v7_flush_dcache_all and get_device_type are declared
in include/asm-arm/arch-omap3/sys_proto.h, so use this file to
declare the functions.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>