arm: mvebu: Add CRS328-4C-20S-4S board

MikroTik CRS328-4C-20S-4S board has a switch chip with an integrated
Marvell Prestera 98DX3236 CPU.

This commit includes two board variants, namely the factory
default one and a Bit variant. The Bit board variant has a
bigger Macronix flash.

Add basic U-Boot, UART and SPI flash support.

Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Jakov Petrina <jakov.petrina@sartura.hr>
Reviewed-by: Stefan Roese <sr@denx.de>