ext: Modified lpc54114 startup code from mcux for use with Zephyr.

The instructions performed by standard Zephyr startup files are removed
from the file startup_LPC54114_cm4.S. Introduced the section
_PlatformInit which will be called when platform specific initialization
is needed.

Signed-off-by: Stanislav Poboril <stanislav.poboril@nxp.com>