dt-bindings: mfd: syscon: Add nuvoton,wpcm450-shm

The Shared Memory interface (SHM) is a piece of hardware in Nuvoton BMCs
that allows a host processor (connected via LPC) to access flash and RAM
that belong to the BMC. The SHM includes a register block accessible from
the BMC side.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221105185911.1547847-5-j.neuschaefer@gmx.net