KVM: arm64: Move ACTLR_EL1 emulation to the sys_reg_descs array

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

reset_actlr() used to write directly to the sys_regs[], now it uses
the vcpu accessor for concistency.

Signed-off-by: James Morse <james.morse@arm.com>