serial: uniphier: set clock rate without clock-frequency property

In Linux, the clock rate of the UART is given by the clock driver.

If you try to follow that in U-Boot, you would end up with adding
more u-boot,dm-pre-reloc properties, and also the clock driver would
be too big for SPL, which is used for UniPhier ARMv7 platform.

The current solution is to add 'clock-frequency' property to the
UART nodes, but it does not exist in the DT files in Linux.  I do
not want to let DT diverge for U-Boot.

Check the SoC compatible and set the clock rate according to it.
This will be helpful to sync DT between Linux and U-Boot.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>