dt-bindings: arm: apple: Add iMac (24-inch 2021) to Apple bindings

This introduces compatible strings for both 2021 Apple iMac M1 devices:

* apple,j456 - iMac (24-inch, 4x USB-C, M1, 2021)
* apple,j457 - iMac (24-inch, 2x USB-C, M1, 2021)

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Janne Grunau <j@jannau.net>
Tested-by: Hector Martin <marcan@marcan.st>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>