arm: mvebu: helios4: Enable SPI flash support

Enable SPI flash support under U-Boot and SPL. The ENV size and offset,
ported from U-Boot 2013.01 Marvell version: 2015_T1.0p16

To create U-Boot image for SPI flash, user would need to replace
* CONFIG_MVEBU_SPL_BOOT_DEVICE_MMC with CONFIG_MVEBU_SPL_BOOT_DEVICE_SPI
* CONFIG_ENV_IS_IN_MMC with CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Aditya Prayoga <aditya@kobol.io>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-By: Dennis Gilmore <dgilmore@redhat.com>
Reviewed-By: Dennis Gilmore <dgilmore@redhat.com>
Signed-off-by: Stefan Roese <sr@denx.de>