drivers: i2c: rename I2C_CLOCK_SPEED Kconfig option

I2C_CLOCK_SPEED Kconfig option is DW driver specific. It does not
define I2C interface speed but rather the I2C DW module clock speed.
It is confusing for a user of any other I2C driver than DW.
This patch renames this option to I2C_DW_CLOCK_SPEED and makes it
visible only for DW I2C driver.

Change-Id: I97f57332fd5cca644eabdef0968a0b2174b885ff
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>