arm: support +CDECP<n> options on cortex-m85

The Cortex-m85 CPU supports the CDE extension, which requires use of the
+cdecp<n> CPU name modifiers.  This patch enables these options.  This is
all pretty-much boiler-plate since Srinath added support on Cortex-m55.

gcc/ChangeLog:

	* config/arm/arm-cpus.in (cortex-m85): Allow +cdecp<n>.
	* doc/invoke.texi: Document this

gcc/testsuite/ChangeLog:

	* gcc.target/arm/multilib.exp: Test CDE options on cortex-m85.