KVM: ARM: Demux CCSIDR in the userspace API

The Cache Size Selection Register (CSSELR) selects the current Cache
Size ID Register (CCSIDR).  You write which cache you are interested
in to CSSELR, and read the information out of CCSIDR.

Which cache numbers are valid is known by reading the Cache Level ID
Register (CLIDR).

To export this state to userspace, we add a KVM_REG_ARM_DEMUX
numberspace (17), which uses 8 bits to represent which register is
being demultiplexed (0 for CCSIDR), and the lower 8 bits to represent
this demultiplexing (in our case, the CSSELR value, which is 4 bits).

Reviewed-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>