arch/arm: add big.LITTLE cpu variants

arch/arm: add big.LITTLE cpu variants

The big.LITTLE configurations can be optimised for by gcc, and a few
users wonder what they should choose when they have such CPUs.

Add new entries for those big.LITTLE configurations.

Note: the various combos were added in various gcc versions, but only
really worked in later versions:

    Variant   | Introduced in | First built in
    ----------+---------------+----------------
    a15-a7    | 4.9           | 4.9
    a17-a7    | 5             | 5
    a57-a53   | 4.9           | 6
    a72-a53   | 5             | 6

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>