rockchip: Add Single boot image (with binman, pad_cat)

All rockchip platforms support TPL or SPL-based bootloader
in mainline with U-Boot proper as final stage. For each
stage we need to burn the image on to flash with respective
offsets.

This patch creates a single boot image component using
- binman, for arm32 rockchip platforms
- pad_cat, for arm64 rockchip platforms.

This would help users to get rid of burning different
boot stage images.

The new image called 'u-boot-rockchip.bin'
which can burn into flash like:

₹ sudo dd if=u-boot-rockchip.bin of=/dev/sda seek=64

This would support all rockchip platforms, except rk3128
since it doesn't support for SPL yet.

Cc: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Cc: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>