arm: Add new config option ARCH_VERY_EARLY_INIT

When this option is set then ARM _main() function would call
arch_very_early_init() function at the beginning. It would be before
calling any other functions like debug_uart_init() and also before
initializing C runtime environment.

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