clk: fix compilation errors for poplar platform

Move clk_release_all() prototype and definition inside
OF_CONTROL flag to avoid following compilation error for
poplar platform:

aarch64:  +   poplar
+drivers/usb/host/built-in.o: In function `ehci_usb_remove':
+drivers/usb/host/ehci-generic.c:159: undefined reference to `clk_release_all'
+drivers/usb/host/built-in.o: In function `ehci_usb_probe':
+drivers/usb/host/ehci-generic.c:133: undefined reference to `clk_release_all'
+make[1]: *** [u-boot] Error 139
+make: *** [sub-make] Error 2

Introduced by 4e542c4 clk: add clk_release_all()

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>