i2c: aspeed: add proper support fo 24xx clock params

24xx BMCs have larger clock divider granularity which can cause problems
when trying to set them as 25xx clock dividers; this adds clock setting
code specific to 24xx.

This also fixes a potential issue where clock dividers were rounded down
instead of up.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>