ANDROID: KVM: arm64: Expose topup_hyp_memcache() to the rest of KVM

In order to simplify the implementation of an EL2-only version of
MMIO guard, expose topup_hyp_memcache() and simplify its usage
by only requiring a vcpu.

While we're at it, make free_hyp_memcache() visible in kvm_host.h

Signed-off-by: Marc Zyngier <maz@kernel.org>
Bug: 209580772
Change-Id: I4f54c57a9693cf7a3450f99fedc15ae32af09a31
Signed-off-by: Will Deacon <willdeacon@google.com>