reset: uniphier: rename MIO reset to SD reset for Pro5, PXs2, LD20 SoCs

I made a mistake as for naming for this block.  The MIO block is not
implemented for these 3 SoCs in the first place.  The current naming
will be a trouble if an SoC with both MIO and SD-ctrl blocks appear
in the future.

This driver has just been merged in the previous merge window.
Rename it before the release.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>