soc: nxp lpc55xxxx: Enable clock on all used Flexcomm ports

Previously the clock was not attached to all Flexcomm ports. Some ports
were configured with FRO_12M, others with FRO_HF, some not at all.

Now every Flexcomm port in I2C, SPI or USART mode uses the FRO_HF clock,
if it is enabled in the device tree, to cover all use cases.

Signed-off-by: Fabian Otto <fabian.otto@rohde-schwarz.com>