clk: meson: Copy meson8b CLKID defines to private header file

Only expose future CLKID constants if necessary. This patch
removes CLK_NR_CLKS from the DT bindings but leaves all previously
defined CLKIDs there to keep backward compatibility.

Signed-off-by: Alexander Müller <serveralex@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1472319654-59048-5-git-send-email-serveralex@gmail.com