soc: mediatek: mtk-dvfsrc: Add a new callback for calc_dram_bw

soc: mediatek: mtk-dvfsrc: Add a new callback for calc_dram_bw

In preparation for adding support for DVFSRC Version 4, add a new
callback for calculating the dram bandwidth, assign the current
calculation algo to all of the currently supported SoCs, and use
this in __dvfsrc_set_dram_bw_v1().

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