uboot-mediatek: add initial Zyxel EX5601-T0 support

uboot-mediatek: add initial Zyxel EX5601-T0 support

Flash procedure is described in next commit.

TLDR:
Copy preloader and uboot to /tmp and write them in the mtd.
This will also require new UBI partition and
 volumes to boot openwrt.

mtd write /tmp/openwrt-mediatek-filogic-zyxel_ex5601-t0-ubootmod-preloader.bin bl2
mtd write /tmp/openwrt-mediatek-filogic-zyxel_ex5601-t0-ubootmod-bl31-uboot.fip fip

Changelist:
 - Added profile for 4k+256 SPI NAND_TYPE
 - Added basic Zyxel EX5601-T0 uboot profile

Backported from hitech95 branch:
 - Button RESET pin fix
 - Button WPS pin fix

Signed-off-by: Valerio 'ftp21' Mancini <ftp21@ftp21.eu>
Signed-off-by: Nicolò Veronese <nicveronese@gmail.com>