arm64: dts: Add Designware I2C controller DTS entries for X-Gene v1 SoC

This patch adds DTS entries for Designware I2C controller used in
APM X-Gene v1 SoC evaluation platform (Mustang board).

APM X-Gene v1 SoC has 2 I2C controllers. On Mustang board,
I2C1 is used to implement proxy I2C interface; I2C1 can be
used as I2C slave port (for BMC) or as I2C master port (if
no BMC is used). Only I2C1 DT entry is added in this
patch with default status as 'disabled'.

Signed-off-by: Duc Dang <dhdang@apm.com>