Add a bus multiplexer device

This patch set adds a bus multiplexer and the necessary infrastructure
in the I2C code to allow it to work.

These are common on systems with lots of I2C devices, like an IPMI BMC.

# gpg: Signature made Thu 13 May 2021 22:48:07 BST
# gpg:                using RSA key FD0D5CE67CE0F59A6688268661F38C90919BFF81
# gpg: Good signature from "Corey Minyard <cminyard@mvista.com>" [unknown]
# gpg:                 aka "Corey Minyard <minyard@acm.org>" [unknown]
# gpg:                 aka "Corey Minyard <corey@minyard.net>" [unknown]
# gpg:                 aka "Corey Minyard <minyard@mvista.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: FD0D 5CE6 7CE0 F59A 6688  2686 61F3 8C90 919B FF81

* remotes/cminyard/tags/for-qemu-6.1-v1:
  hw/i2c: add pca954x i2c-mux switch
  hw/i2c: move search to i2c_scan_bus method
  hw/i2c: add match method for device search
  hw/i2c: name I2CNode list in I2CBus

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>