rockchip: clk: rk3399: fix warnings for unused variables in SPL/non-SPL

Due to differences in the code paths for SPL and non-SPL, some static
constant structures remain unused in each build variant. This raises
warnings with recent GCC versions (we currently use GCC-6.3).

The warnings addressed in this commit (by matching #if conditions for
the variable definition with their uses) are:

* for the SPL build:
    drivers/clk/rockchip/clk_rk3399.c:53:29: warning: 'cpll_init_cfg' defined but not used [-Wunused-const-variable=]
     static const struct pll_div cpll_init_cfg = PLL_DIVISORS(CPLL_HZ, 1, 2, 2);
                                 ^~~~~~~~~~~~~
    drivers/clk/rockchip/clk_rk3399.c:52:29: warning: 'gpll_init_cfg' defined but not used [-Wunused-const-variable=]
     static const struct pll_div gpll_init_cfg = PLL_DIVISORS(GPLL_HZ, 2, 2, 1);
                                 ^~~~~~~~~~~~~
* for the non-SPL build:
    drivers/clk/rockchip/clk_rk3399.c:54:29: warning: 'ppll_init_cfg' defined but not used [-Wunused-const-variable=]
     static const struct pll_div ppll_init_cfg = PLL_DIVISORS(PPLL_HZ, 2, 2, 1);
                                 ^~~~~~~~~~~~~

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Simon Glass <sjg@chromium.org>