kvm/powerpc: rename kvm_hypercall() to epapr_hypercall()

kvm_hypercall() have nothing KVM specific, so renamed to epapr_hypercall().
Also this in moved to arch/powerpc/include/asm/epapr_hcalls.h

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>