clk: boston: Add a driver for MIPS Boston board clocks

clk: boston: Add a driver for MIPS Boston board clocks

Add a driver for the clocks provided by the MIPS Boston board from
Imagination Technologies. 2 clocks are provided - the system clock & the
CPU clock - and each is a simple fixed rate clock whose frequency can be
determined by reading a register provided by the board.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: linux-clk@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16483/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>