ARM: dts: spear3xx: Use plgpio regmap in SPEAr310 and SPEAr320

Resources used by plgpio and pinmux are conflicting on SPEAr310
and SPEAr320.

Use the newly introduced regmap property in plgpio node to use
pinmux resources from plgpio and so avoid the conflict.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://lore.kernel.org/r/20211202095255.165797-5-herve.codina@bootlin.com

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