flash: add Nuvoton NUC910 series support

Not tested, adapted from http://tech.groups.yahoo.com/group/versaloon/message/391



Change-Id: Ic3273e64fd99bffab16764b06227e09a05d07c8f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1510


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Nemui Trinomius <nemuisan_kawausogasuki@live.jp>