platform/x86: mlx-platform: Add physical bus number auto detection

mlx-platform does not provide a bus number to i2c-mlxcpld, assuming it
is always one. On some x86 systems, other i2c drivers may probe before
i2c-mlxcpld, causing bus one to be busy.

Make mlx-platform determine which adapter number is free prior to
activating i2c-mlxpld, adjusting the mux base numbers accordingly.
Update the mlxreg-hotplug pdata similarly.

This adds an explicit mlx-platform build dependency on I2C, update the
Kconfig accordingly. Add the missing REGMAP dependency while we're at
it.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
[dvhart: Rewrite commit message more concisely]
[dvhart: Add build dependencies]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>