i2c: Create common default i2c_[set|get]_bus_speed() functions

New default, weak i2c_get_bus_speed() and i2c_set_bus_speed() functions
replace a number of architecture-specific implementations.

Also, providing default functions will allow all boards to enable
CONFIG_I2C_CMD_TREE.  This was previously not possible since the
tree-form of the i2c command provides the ability to display and modify
the i2c bus speed which requires i2c_[set|get]_bus_speed() to be
present.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>