i2c: mlxbf: I2C_MLXBF should depend on MELLANOX_PLATFORM

The Mellanox BlueField I2C controller is only present on Mellanox
BlueField SoCs.  Hence add a dependency on MELLANOX_PLATFORM, to prevent
asking the user about this driver when configuring a kernel without
Mellanox platform support.

Fixes: b5b5b320 ("i2c: mlxbf: I2C SMBus driver for Mellanox BlueField SoC")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@kernel.org>