include/drivers/clock_control: stm32: Simplify macro definitions

When possible, replace DT_INST(0, st_stm32xx_rcc) by equivalent
DT_NODELABEL(rcc).
Also, replace DT_NODE_HAS_COMPAT_STATUS(DT_NODELABEL(pll),
st_stm32f1_pll_clock, okay) by equivalent
DT_NODE_HAS_STATUS(DT_NODELABEL(pll), okay).

Last, don't check about rcc node label status, which is not an
optional node.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>