ar71xx: add 64kb bootloader mtd parser for tplinkpart

Signed-off-by: Weijie Gao <hackpascal@gmail.com>

This patch adds flash layout parser for TP-Link firmwares which have a 64kb
bootloader.

This is used for TP-Link TL-WDR6500 v2.

SVN-Revision: 46662