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
