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 `clkhwops_omap3430es2_iclk_hsotgusb_wait'
drivers/clk/ti/interface.c:100: undefined reference to `clkhwops_omap3430es2_iclk_dss_usbhost_wait'
drivers/clk/ti/interface.c:100: undefined reference to `clkhwops_omap3430es2_iclk_ssi_wait'
drivers/clk/ti/interface.c:100: undefined reference to `clkhwops_am35xx_ipss_wait'
drivers/built-in.o: In function `ti_clk_register_composite':
:(.text+0x3da768): undefined reference to `ti_clk_build_component_gate'

In order to fix that problem, this patch makes the omap3 legacy code
compiled only when both CONFIG_OMAP3 and CONFIG_ATAGS are set.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>