dt-bindings: i2c: mpc: use proper binding for transfer timeouts

"i2c-scl-clk-low-timeout-us" has flaws in itself and the usage here is
all wrong. The driver doesn't use it as a maximum time for clock
stretching but the maximum time for a total transfer. We already have
a binding for the latter. Convert the wrong binding from examples.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>