SPCI: Add a buffer of len PAGE_SIZE to each CPU.

The buffers are used to store a copy of the contents of the Tx buffer
when handling an architected message. This is mostly relevant for
memory sharing.
There exists a single buffer per CPU.

Change-Id: I60c4eab865b01fb73825308439faa72c8af85fb2
diff --git a/inc/hf/cpu.h b/inc/hf/cpu.h
index d44d68e..56f7982 100644
--- a/inc/hf/cpu.h
+++ b/inc/hf/cpu.h
@@ -119,6 +119,8 @@
 bool cpu_on(struct cpu *c, ipaddr_t entry, uintreg_t arg);
 void cpu_off(struct cpu *c);
 struct cpu *cpu_find(cpu_id_t id);
+uint8_t *cpu_get_buffer(cpu_id_t cpu_id);
+uint32_t cpu_get_buffer_size(cpu_id_t cpu_id);
 
 struct vcpu_locked vcpu_lock(struct vcpu *vcpu);
 void vcpu_unlock(struct vcpu_locked *locked);