arch/arm/src/max326xx/max32660/max32660_serial.c: Add a mostly commented out serial driver. With this, we can accomplish a complete build with many warnings for 'Missing logic'
configs/max32660-evsys/src/max326_button.c: Add support for the on-board button.
arch/arm/src/max326xx: Add a mostly empty max326_lowputc.c file mostly so that we can get further in the compilation. Fixed several more compile errors revealed by this.
arch/arm/src/max326xx: Add peripheral clock control header file.
