net: mdio: mscc-miim: Add depend of REGMAP_MMIO on MDIO_MSCC_MIIM

There's build error while CONFIG_REGMAP_MMIO is not set
and CONFIG_MDIO_MSCC_MIIM=m.

ERROR: modpost: "__devm_regmap_init_mmio_clk"
[drivers/net/mdio/mdio-mscc-miim.ko] undefined!

Add the depend of REGMAP_MMIO to fix it.

Fixes: a27a7628 ("net: mdio: mscc-miim: convert to a regmap implementation")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20211130110209.804536-1-dtcccc@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>