clk: omap: compile legacy omap3 clocks conditionally

The 'ARM: OMAP3: legacy clock data move under clk driver' patch series
causes build errors when CONFIG_OMAP3 is not set:

drivers/clk/ti/dpll.c: In function 'ti_clk_register_dpll':
drivers/clk/ti/dpll.c:199:31: error: 'omap3_dpll_ck_ops' undeclared (first use in this function)
  const struct clk_ops *ops = &omap3_dpll_ck_ops;
                               ^
drivers/clk/ti/dpll.c:199:31: note: each undeclared identifier is reported only once for each function it appears in
drivers/clk/ti/dpll.c:259:10: error: 'omap3_dpll_per_ck_ops' undeclared (first use in this function)
   ops = &omap3_dpll_per_ck_ops;
          ^

drivers/built-in.o: In function `ti_clk_register_gate':
drivers/clk/ti/gate.c:179: undefined reference to `clkhwops_omap3430es2_dss_usbhost_wait'
drivers/clk/ti/gate.c:179: undefined reference to `clkhwops_am35xx_ipss_module_wait'
-in.o: In function `ti_clk_register_interface':
drivers/clk/ti/interface.c:100: undefined reference to `clkhwop...