uboot-lantiq: Add BT Home Hub 5A support

Based on the submission to the uboot-lantiq repo by Martin Blumenstingl.

Use the ddr_settings.h from the GPL tarball. The NAND boot optimized
one (with memory tuning enabled) doesn't work for the UART boot image.

Use the same mtd layout as the stock u-boot. Add add UBI support.

Use the leds to indicate boot status like it is done with the stock
u-boot. Switch on the red power led if kernel image can't be loaded.
Otherwise switch the green led on.

Make only the ramboot u-boot available. Only this image is required for
the first installation of LEDE.

Signed-off-by: Mathias Kresin <dev@kresin.me>