CONFIG_SOCFPGA_DWMMC_FIFO_DEPTH is defined in the socfpga_common.h,
but not referenced at all.  Remove.

Also, clean-up the README.socfpga.  CONFIG_MMC should not be defined
in the header since it was moved to Kconfig by commit c2726995
("mmc: complete unfinished move of CONFIG_MMC").  I see no grep hit
for the others.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NMarek Vasut <marex@denx.de>