arm: mvebu: Avoid reading MVEBU_REG_PCIE_DEVID register too many times

Change detection of platform/cpu from runtime to compile time via config
define. This completely eliminates compiling code which is not going to run
on selected platform. Code which parses and prints device / revision id
still reads device id from MVEBU_REG_PCIE_DEVID register, but only once.

Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>