units: Add HZ_PER_GHZ

The is going to be a new user of the HZ_PER_GHZ definition besides
possibly existing ones. Add that one to the header.

While at it, split Hz and kHz groups of the multipliers for better
maintenance and readability.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260112134900.4142954-2-andriy.shevchenko@linux.intel.com