davinci: fix compile warning: <mach/da8xx.h>: #include <linux/platform_device.h>

This hushes the following warning:

arch/arm/mach-davinci/include/mach/da8xx.h:104: warning: ‘struct platform_device’
declared inside parameter list
arch/arm/mach-davinci/include/mach/da8xx.h:104: warning: its scope is only this
definition or declaration, which is probably not what you want

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>