ARM: pxa: move smemc register access from clk to platform

The get_sdram_rows() and get_memclkdiv() helpers need smemc
register that are separate from the clk registers, move
them out of the clk driver, and use an extern declaration
instead.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-clk@vger.kernel.org
Link: https://lore.kernel.org/lkml/87pnielzo4.fsf@belgarion.home/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>