arm64: mm: Rename internal split_pmd API to not conflict with upstream

Upstream commit "da141706 arm64: add better page protections to arm64"
added a split_pmd API which is different that the one used in
CONFIG_FORCE_PAGES, an internal debug feature. Rename the internal API
to allow the internal debug feature to compile and work as expected.

Change-Id: I0d4bc7d1a7fe22c1e4964662cc3820db4e247c38
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>