ANDROID: KVM: arm64: Add a helper to convert kern VA into a pKVM mod VA

Just like pkvm_el2_mod_va() helps modules to convert a kernel VA address
into pKVM mod accessible VA, add a helper accessible from the kernel
code.

Bug: 455467611
Bug: 333671007
Bug: 357781595
Change-Id: I63ee8e33818f219a26c3e355df4817ca5c944cdd
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>