arm: Fix the wrong error message string for mve vldr/vstr (PR26763).

arm: Fix the wrong error message string for mve vldr/vstr (PR26763).

For mve vldr/vstr instructions assembler is throwing wrong error message.
Instead of 'Error: syntax error' assembler fails with 'Error: lo register required'.
This patch fixes the issue.

eg:
$ cat x.s
.syntax unified
.thumb

vldrb.s16 q0, r0

Before this patch:
$ arm-none-eabi-as x.s -march=armv8.1-m.main+mve -mfloat-abi=hard
x.s: Assembler messages:
x.s:4: Error: lo register required -- `vldrb.s16 q0,r0'

After this patch:
$ arm-none-eabi-as x.s -march=armv8.1-m.main+mve -mfloat-abi=hard
x.s: Assembler messages:
x.s:4: Error: syntax error -- `vldrb.s16 q0,r0'

gas/ChangeLog:

2020-10-21  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

	PR target/26763
	* config/tc-arm.c (parse_address_main): Add new MVE addressing mode
	check.
	* testsuite/gas/arm/mve-vldr-vstr-bad.d: New test.
	* testsuite/gas/arm/mve-vldr-vstr-bad.l: Likewise.
	* testsuite/gas/arm/mve-vldr-vstr-bad.s: Likewise.