arm: PR gas/25472 Enable DSP instructions with +mve

arm: PR gas/25472 Enable DSP instructions with +mve

We noticed +mve was not enabling DSP instructions as it should, reported in PR
25472.
The MVE architecture extension for Armv8.1-M Mainline implies DSP extensions.
This patch reflects that in the '+mve' command line option.

gas/ChangeLog:
2020-01-31  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	PR gas/25472
	* config/tc-arm.c (armv8m_main_ext_table): Refactored +dsp adding.
	(armv8_1m_main_ext_table): Refactored +dsp adding and enabled dsp for
	+mve.
	* testsuite/gas/arm/mve_dsp.d: New test.