arm64: Fix the capitalization of CurrentEL

[andrew]

Summary

Use the same capitalization as the Arm documentation.

Sponsored by: Arm Ltd

[kajetan.puchalski_arm.com]

There is one lowercase 'currentel' in the machdep file, though in a block of all-lowercase registers so maybe we don't want to capitalise that one. LGTM otherwise.

sys/arm64/arm64/machdep.c
1000:	PRINT_REG(currentel);

[andrew]

That code probably needs to be updated, e.g. it's missing some ID registers