mm: cma: restrict base_pfn exposure

We are exposing cma region's base_pfn without any restriction. Make this
dependent on kptr_restrict configuration.

Change-Id: If454194cc5e7639f33b2e1dd29f5d925a17340c1
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>