mmc: uniphier: Split out SoC specific bits from the driver

Factor out common code from the uniphier SD driver, change the prefix
of the functions from uniphier_sd_ to matsu_sd_ and create separate
renesas-sdhi.c driver. Thus far, all the code is still compiled when
CONFIG_UNIPHIER_MMC is selected and there is no functional change.
This patch is a preparation for further split of the SoC specific
parts of the Matsushita SD driver, used both on Uniphier and R-Car.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>