rockchip: px30: Define variables for compressed image support

The standard boot path expects the kernel_comp_addr_r and kernel_comp_size
variables for booting compressed kernel images. Define them using the previous
kernel_addr_c value (likely initially meant for this purpose) and usual size.

This was tested on the PX30 EVB to successfully boot compressed Linux kernel
images.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>