i2c: xlp9xx: add ACPI support for Broadcom Vulcan

Added ACPI support for the I2C controller present on Broadcom's
Vulcan ARM64 processor. ACPI ID used by the controller is BRCM9007.

Changed the xlp9xx_i2c_get_frequency() function to use
device_property_read_u32() API so that the "clock-frequency" value
can be read from _DSD in ACPI mode.

Signed-off-by: Tanmay Jagdale <tanmay.jagdale@broadcom.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>