dt-bindings: mmc: Add bindings for LiteSDCard

LiteSDCard is a small footprint, configurable SDCard core for
FPGA based SoCs.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220113170300.3555651-3-gsomlo@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>