[ARM] Add support for Cortex-A55 and Cortex-A75.

[ARM] Add support for Cortex-A55 and Cortex-A75.

This patch adds support for the ARM Cortex-A55 and
Cortex-A75 processors.

The ARM Cortex-A55 and Cortex-A75 procsessors implement the ARMv8-A
architecture, with support for the ARMv8.1-A and ARMv8.2-A extensions,
including support for the 16-bit floating point extensions.

The 16-bit floating-point extensions are optional, and we haven't defined
an option mapping straight to them thus far, so this patch first needs to
add one of those in include/opcode/arm.h, then we can simply add the CPU names
as usual in config/tc-arm.c .

Tested on arm-none-eabi.

2017-07-05  James Greenhalgh  <james.greenhalgh@arm.com>

	* config/tc-arm.c (arm_cpus): Add Cortex-A55 and Cortex-A75.
	* doc/c-arm.texi (-mcpu): Document Cortex-A55 and Cortex-A75.