ARM: mstar: Add mpll to base dtsi

All of the currently known MStar/SigmaStar ARMv7 SoCs have at least
one MPLL and it seems to always be at the same place so add it to
the base dtsi.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20210301123542.2800643-4-daniel@0x0f.com

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