sys/types.h: include stdint.h to get uint32_* types
debug: add function headers and debug_print for debugging

Add platform_{dputc, dgetc} function headers and implement debug_print
that sends strings to platform_dputc.
register.h: add macro for accessing memory mapped registers
realview-pb: add support for realview-pb
top: use debug_print for printing welcome text

Use debug_print and add lib/debug dependency