cpaccess: remove use of set_get_l2_indirect_reg()

cpaccess is the only client of set_get_l2_indirect_reg(), a special-
purpose API for writing an indirect Krait CP15 register and reading
back the new value. For simplicity, this API is being removed and
replaced with bacl-to-back calls to set_l2_indirect_reg() and
get_l2_indirect_reg(), which should perform comparably.

Change-Id: I868ae115265c59e58ffd37dc405a91c6962e0c3d
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>