i2c: mlxbf: Update reference clock frequency

The reference clock frequency remains the same across Bluefield
products. Thus, update the frequency and rename the macro.

Fixes: b5b5b320 ("i2c: mlxbf: I2C SMBus driver for Mellanox BlueField SoC")
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Khalil Blaiech <kblaiech@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>