ARM: mstar: Add Armv7 base dtsi

Adds initial dtsi for the base MStar/Sigmastar Armv7 SoCs.

These SoCs have very similar memory maps and this will avoid
duplicating nodes across multiple dtsis.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>