arm: mediatek: add support for MediaTek MT7987 SoC

arm: mediatek: add support for MediaTek MT7987 SoC

This patch adds basic support for MediaTek MT7987 SoC.
This includes files that will initialize the SoC after boot and
its device tree.

In order to maximize the continuous usable  memory space, MT7987 has its
ATF BL31 loaded at the top of RAM. Since u-boot will also locate itself to
top of RAM, u-boot will read the actual memory region of BL31 and set
correct gd->ram_top to avoid u-boot overlapping with BL31.

As now support for mt7987 hasn't been submitted to linux kernel, all dts
filed will be put to arch/arm/dts. They'll be removed after successfully
being merged by linux kernel, and OF_UPSTREAM will also be switched on.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>