dt-bindings: i3c: Add AST2600 i3c controller

Add a devicetree binding for the ast2600 i3c controller hardware. This
is heavily based on the designware i3c core, plus a reset facility
and two platform-specific properties:

 - sda-pullup-ohms: to specify the value of the configurable pullup
   resistors on the SDA line

 - aspeed,global-regs: to reference the (ast2600-specific) i3c global
   register block, and the device index to use within it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org> (on v1)
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20230331091501.3800299-3-jk@codeconstruct.com.au
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>