clk: boston: Providea simple driver for Boston board clocks

clk: boston: Providea simple driver for Boston board clocks

Add a simple driver for the clocks provided by the MIPS Boston
development board. The system provides information about 2 clocks whose
rates are fixed by the bitfile flashed in the boards FPGA, and this
driver simply reads the rates of these 2 clocks.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Simon Glass <sjg@chromium.org>