drivers: clock_control: fix the CH32V00x clock definitions

Some time during the initial review, the CH32V003 clock definitions
were exended to include clocks from the CH32V20x and CH32V30x series.
As the CH32V20x series now has a separate clocks include, remove the
non-existent clocks.

While there, drop the 'A' prefix to match the register names in the
CH32V00X reference manual.

Also add the USART2 clock that's present in the CH32V00x series.

Signed-off-by: Michael Hope <michaelh@juju.nz>