Blackfin: serial: move early debug strings into .rodata section

Rewrite the assembly serial_early_puts() helper to place the strings
in the .rodata section rather than embedding them directly in the
.text section.  Using .text is a little simpler, but it doesn't let
people execute out of internal L1 sram (since core reads don't work
on those regions).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>