arm: mvebu: helios4: Enable SPI flash support

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 <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Tested-By: Dennis Gilmore <[email protected]>
Reviewed-By: Dennis Gilmore <[email protected]>
Signed-off-by: Stefan Roese <[email protected]>