KVM: arm64: Remove __vcpu_{read,write}_sys_reg_{from,to}_cpu()

KVM: arm64: Remove __vcpu_{read,write}_sys_reg_{from,to}_cpu()

There is no point having __vcpu_{read,write}_sys_reg_{from,to}_cpu()
exposed to the rest of the kernel, as the only callers are in
sys_regs.c.

Move them where they below, which is another opportunity to
simplify things a bit.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250817121926.217900-5-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>