mx8mm: use cortexa53-crypto tune for imx8m mini per default
Current implementation of i.MX8M Mini is utilizing generic aarch64 tune
(arch-arm64) provided from OE meta layer.
According to IMX8MMRM [1] this CPU contains Cortex-A53 cores, so this commit
sets a default tune to cortexa53-crypto.
Note, that according to [1] ARM Crypto extensions are supported, therefore it
is beneficial to set the tune to -crypto, as this also enables NEON and VFP
support in GCC (via -march and -mcpu feature modifiers, see [2]).
