arch/arm: add two new cortex-based armv8.2a cores

arch/arm: add two new cortex-based armv8.2a cores

The cortex-a76 implements the full amrv8.2a extensions, and some
optional extensions from the armv8.3a, armv8.4a, and armv8.5a sets,
but none of their mandatory extensions, which means that it does not
qualify for better than an armv8.2a:
    https://developer.arm.com/products/processors/cortex-a/cortex-a76
    http://infocenter.arm.com/help/topic/com.arm.doc.100798_0301_00_en/giq1479805174793.html
    http://infocenter.arm.com/help/topic/com.arm.doc.100798_0301_00_en/fjv1477559794375.html

Also, gcc fits it in the armv8.2a category, too:
    https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/aarch64/aarch64-cores.def;h=67ce42fb8aacd4c246295f32151a03b1f318ae44;hb=HEAD#l97

Build tested:
https://gitlab.com/kubu93/buildroot/pipelines/60318953

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>