arch/arm: add the cortex A17 variant supported by gcc 5.x

Add the Cortex A17 variant. This core is considered a replacement
of the Cortex A12 and is supported by gcc 5 / binutils 2.25+

Suggested-by: Ross Green <greenfross@netscape.net>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>