parallel.c: Consoldiate parallel master registration logic

This is analogous to spi.c and opaque.c however parallel
logic was previously never consoldiated.

This free's up flashrom.c from namespace pollution.

BUG=b:242246291
TEST=builds with both make and meson.

Change-Id: Ie08e2e6c51ccef5281386bf7e3df439b91573974
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/66651
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>

meson.build

@@ -97,6 +97,7 @@ srcs = files(
   'layout.c',
   'libflashrom.c',
   'opaque.c',
+  'parallel.c',
   'print.c',
   'programmer.c',
   'programmer_table.c',