arm64: dts: mediatek: add dtsi for MT8516

The MT8516 SoC provides the following peripherals: GPIO, UART, USB2,
SPI, eMMC, SDIO, NAND, Flash, ADC, I2C, PWM, Timers, IR, Ethernet, and
Audio (I2S, SPDIF, TDM).

This commit is adding the basic dtsi file with the support of the
following IOs: GPIO, UART, SPI, eMMC, I2C, Timers.
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>