dt-bindings: mmc: samsung,exynos-dw-mshc: add specific compatible for exynos8890

Add samsung,exynos8890-dw-mshc-smu specific compatible to the bindings
documentation. Since Samsung, as usual, likes reusing devices from older
designs, use the samsung,exynos7-dw-mshc-smu compatible.

Signed-off-by: Ivaylo Ivanov <ivo.ivanov.ivanov1@gmail.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>