soc: mediatek: mtk-dvfsrc: Write bandwidth to EMI DDR if present

soc: mediatek: mtk-dvfsrc: Write bandwidth to EMI DDR if present

In preparation for adding support for DVFSRC Version 4, add a new
`has_emi_ddr` member to struct dvfsrc_soc_data: if true, write the
DRAM bandwidth both to the BW_AVG and to the newly defined EMI_BW
register, present only on DVFSRC v4.

Currently supported SoCs will not use this, as has_emi_ddr is left
out from their platform data, hence reading false.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>