arm: mvebu: Add CRS305-1G-4S Bit board

MikroTik CRS305-1G-4S Bit board has a switch chip with an integrated
Marvell Prestera 98DX3236 CPU.

The Bit board variant is added, which has a bigger Macronix flash.

Add basic U-Boot, UART and Winbond 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>