CV32E40Pv2 SIMD 6-bit Unsigned Immediate

	Created a 6-bit unsigned immediate under the operand name 'b8'
	for SIMD instructions that require this immediate.

	* gas/config/tc-riscv.c: Added operand name 'b8' for 6-bit unsigned
	  immediate for SIMD.
	* include/opcode/riscv.h: Created ENCODED_CV_SIMD_UIMM6 and
	  EXTRACT_CV_SIMD_UIMM6.
	* opcodes/riscv-dis.c: Added disassemble information for 'b8'.

Signed-off-by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>