ARM: dts: lpc32xx: set default clock rate of HCLK PLL

Probably most of NXP LPC32xx boards have 13MHz main oscillator and
therefore for HCLK PLL and ARM core clock rate default hardware
setting is 16 * 13MHz = 208MHz, however a user may vary HCLK PLL/ARM
core rate from 156MHz to about 266MHz for 13MHz clock source.

The change explicitly defines HCLK PLL output rate to default 208MHz
to overwrite any settings done by a bootloader, if needed it can be
redefined in a board DTS file.

Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>