clk: lpc18xx: add NXP specific COMMON_CLK_NXP configuration symbol

The change adds COMMON_CLK_NXP configuration symbol and enables it for
NXP LPC18XX architecture, this is needed to reuse drivers/clk/nxp
folder for NXP common clock framework drivers other than LPC18XX one.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>