configs/rockpro64: new defconfig

configs/rockpro64: new defconfig

A defconfig for the rockpro64 was added in
59e1077b4ebe79622a454b0a8a840e4349a0440d, but then removed
in 10f49213b7e2837ae6ed63fc16fbdaa954ecd87f because an ARM32
compiler was needed to build ATF, and this was not supported
back then.

Now, since package/arm-gnu-a-toolchain/ has been added, this
defconfig can be re-introduced.

Following new changes are introduced in comparision with the
older changeset:

    - renamed defconfig from pine64_rockpro64 to rockpro64
    - updated readme
    - using a GPT partition layout
    - update defconfig to build ATF
    - bump u-boot to 2020.07
    - bump linux to 5.8.2

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout: fixate U-Boot version; remove blind BR2_TARGET_UBOOT_NEEDS_PYTHON]