KVM: arm64: Move ACTLR_EL1 emulation to the sys_reg_descs array

The only entry in the genericv8_sys_regs arrays is for emulation of
ACTLR_EL1. As all targets emulate this in the same way, move it to
sys_reg_descs[].

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200622113317.20477-4-james.morse@arm.com