armv8: always use current exception level for TCR_ELx access

Currently get_tcr() takes an "el" parameter, to select the proper
version of the TCR_ELx system register.
This is problematic in case of the Apple M1, since it runs with
HCR_EL2.E2H fixed to 1, so TCR_EL2 is actually using the TCR_EL1 layout,
and we get the wrong version.

For U-Boot's purposes the only sensible choice here is the current
exception level, and indeed most callers treat it like that, so let's
remove that parameter and read the current EL inside the function.
This allows us to check for the E2H bit, and pretend it's EL1 in this
case.

There are two callers which don't care about the EL, and they pass 0,
which looks wrong, but is irrelevant in these two cases, since we don't
use the return value there. So the change cannot affect those two.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Mark Kettenis <kettenis@openbsd.org>