mmc: uniphier-sd: Add compatible strings for RCar Gen2

Add DT compatible strings for RCar Gen2 SoCs, so that this driver
can bind with them. Unlike Gen3, which uses 64bit FIFO, the Gen2
uses 16bit FIFO.

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