i2c: xlp9xx: Get clock frequency with clk API

Get the input clock frequency to the controller from the linux clk
API, if it is available. This allows us to pass in the block input
frequency either from ACPI (using APD) or from device tree.

The old hardcoded frequency is used as default for backwards compatibility.

Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
Signed-off-by: Kamlakant Patel <kamlakant.patel@cavium.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>