dt-bindings: pinctrl: describe sa8775p-tlmm

Add DT bindings for the TLMM controller on sa8775p platforms.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230201150011.200613-2-brgl@bgdev.pl
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>