gdb: new command 'maint flush dcache'

gdb: new command 'maint flush dcache'

Add a new command to flush the dcache.

gdb/ChangeLog:

	* NEWS: Mention new commands.
	* target-dcache.c: Add 'cli/cli-cmds.h' include.
	(maint_flush_dcache_command): New function.
	(_initialize_target_dcache): Create new 'maint flush dcache'
	command.

gdb/doc/ChangeLog:

	* gdb.texinfo (Caching Target Data): Document 'maint flush
	dcache'.

gdb/testsuite/ChangeLog:

	* gdb.base/dcache-flush.c: New file.
	* gdb.base/dcache-flush.exp: New file.