i2c: designware: dw_i2c_init_driver as subsys initcall

There are few drivers which are available on i2c bus but have been initialized
with subsys_initcall. Also as I2C is a bus driver, it should be available as
early as possible.

Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@st.com>

[wsa: Slightly updated the commit message]

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>