ARM: mstar: Add common dtsi for SSD201/SSD202D

The SSD201 and SSD202D are basically the same chip with a different DDR die
packaged (64MB DDR2 or 128MB DDR3).

This patch adds a shared dtsi for the common parts of these chips like
gpio, pinctrl etc.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20201201134330.3037007-6-daniel@0x0f.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>