riscv: dts: unmatched: Add gpio card detect to mmc-spi-slot

riscv: dts: unmatched: Add gpio card detect to mmc-spi-slot

Per HiFive Unmatched schematics, the card detect signal of the
micro SD card is connected to gpio pin #15, which should be
reflected in the DT via the <gpios> property, as described in
Documentation/devicetree/bindings/mmc/mmc-spi-slot.txt.

[1] https://sifive.cdn.prismic.io/sifive/6a06d6c0-6e66-49b5-8e9e-e68ce76f4192_hifive-unmatched-schematics-v3.pdf

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Fixes: 89656b4ad2d7 ("riscv: dts: add initial board data for the SiFive HiFive Unmatched")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>