add timer_src_clk_sys feature to support running RP2350x timer at s…

[U007D]

…ystem clock frequency

NOTE:
@Dirbaio,

The changes seem to work. But I'm concerned that removing the tick-hz-1_000_000 from the time-driver feature may break existing users. What do you think?

I'll submit the MR, with this concern expressed in the description.

[lulf]

…ystem clock frequency

NOTE: @Dirbaio,

The changes seem to work. But I'm concerned that removing the tick-hz-1_000_000 from the time-driver feature may break existing users. What do you think?

The default for not setting this feature is defined by the docs to be 1 mhz (https://docs.embassy.dev/embassy-time-driver/git/default/index.html#tick-rate), so I think this should be OK.

I'll submit the MR, with this concern expressed in the description.

[U007D]

This MR now also properly sets the RP2350's TIMER0.SOURCE to CLK_SYS on init() when the timer-src-clk-sys feature is enabled.