bcm53xx: Add support for D-Link DIR-890L

bcm53xx: Add support for D-Link DIR-890L

The DIR-890L is very similar to DIR-885L, but has both USB2
and USB3. The signature for the wrgac36 board was copied from
DD-Wrt.

The DIR-890L bootstrap will only load the first 2 MB after
the SEAMA header in the NAND flash, uncompress it with LZMA
and execute it. Since the compressed kernel will not fit in
2 MB we have a problem. Solve this by putting a LZMA
compressed U-Boot into the first 128 KB of the flash
followed by the kernel. The bootstrap will then uncompress
and execute U-Boot and then we let U-Boot read the kernel
from flash and execute it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>