arm64: Rename drop_to_el1 to enter_kernel_el

In the future we may not drop to EL1, e.g. when we support FEAT_VHE
where the kernel runs in EL2.

Reviewed by: emaste, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43976

(cherry picked from commit 801160f4c0a3afbd136a1a6402c9ca233dfdc1eb)