wan: ixp4xx_hss: prepare compile testing

The ixp4xx_hss driver needs the platform data definition and the
system clock rate to be compiled. Move both into a new platform_data
header file.

This is a prerequisite for compile testing, but turning on compile
testing requires further patches to isolate the SoC headers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>