arm: imx: Fix a build error redefinition of 'request_bus_freq'

Since commit 8e453ad3b1d2("ARM: imx: add i.MX7D bus-freq
support") only add i.MX7D bus-freq support, and refer
to 864c1631823f("add imx6ul bus-freq"), but there is no
Enable HAVE_IMX_BUSFREQ for imx6 platform.

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>