This adds basic architectural support for the LPC546xx family and includes...

This adds basic architectural support for the LPC546xx family and includes support for the LPCXpresso-LPC54628 board.  The basic NSH port is almost complete... still lacking GPIO support and LED support.  There are still no significant drivers available.

Squashed commit of the following:

    arch/arm/src/lpc54xx:  Finish off some missing logic.  Complete now execpt for GPIO and LED support.
    arch/arm/src/lpc54xx:  Add lpc54_clrpend.c
    arch/arm/src/lpc54xx:  Serial driver is complete and compiles.
    arch/arm/src/lpc54xx:  Add beginning of a serial driver (still missing some logic)
    arch/arm/src/lpc54xx:  Fleshes out low level USART intialization.
    arch/arm/src/lpc546xx/Kconfig: Add hooks to integrate with common seril upper half.
    arch/arm/src/lpc54xx:  Beginning of USART console support.
    arch/arm/src/lpc54xx: Completes very basic clock configuration.
    arch/arm/src/lpc54xx:  Add clocking logic (still not complete)
    arch/arm/src/lpc54xx:  Beginning of PLL configuration logic.
    arch/arm/src/lpc54xx:  Fix a few things from first compile attempt.  Compilation cannot work until I at least finish the clock configuration logic.
    arch/arm/src/lpc54xx: Addes some SysTick logic.
    arch/arm/src/lpc54xx:  Completes basic startup logic (sans clock configuration) and interrupt configuration.
     arch/arm/src/lpc54xx:  Add generic ARMv7-M start-up logic (needs LPC54628 customizations); add emtpy file that will eventually hold the clock configuration logic.
    arch/arm/src/lpc54xx:  Add (incomplete) SYSCON register definition header file.
    arch/arm/src/lpc54xx:  Add FLEXCOMM header file.
    arch/arm/src/lpc54xx:  Bring in tickless clock logic from LPC43; configs/lpcxpresso-lpc54628: mount procfs if enabled.
    arch/arm/src/lpc54xx: Add RIT clock definitions; add SysTick initialization (not finished)
    LPC54xx and LPCXpresso-LPC54628: add more boilerplate files and stubbed out files.
    arch/arm/src/lpc54xx:  Add (incomplete) USART header file.
    Add another condition to a Kconfig; refresh a defconfig.
    arch/arm/src/lpc54xx/chip: Add LPC54628 memory map header files.
    configs/lpcxpresso-lpc54628:  Add basic build files for the LPCXpresso-LPC54628
    arch/: Basic build directory structure for the LPC54628