LXT/HXT are external crystl oscillator and can be absent on custom board.
This enables configuring LXT/HXT presence:
1. By default, LXT/HXT are configured to be present, except M252 which has no HXT.
2. When LXT is configured to not present, lp_ticker/watchdog will clock by LIRC instead.
3. Limitations:
(1) On all targets, LIRC-clocked lp_ticker gets inaccurate and fails to pass tests.
(2) On NUC472/M453, HIRC-clocked PLL doesn't output 1MHz-aligned frequency. us_ticker gets slight inaccurate.
(3) On all targets, LIRC-clocked RTC is not supported due to no H/W path/RTC clock source reset to LXT on reset/RTC not trimmed for other clock rates.
4. On M263, TRNG's clock source defaults to LXT and needs special handling without LXT.
5. On M252, replace target.hxt-enable with target.hxt-present for consistency.
Consider the following factors to define WDT reset delay:
1. Cannot be too small. This is to avoid premature WDT reset in pieces of timeout cascading.
2. Cannot be too large. This is to pass Greentea reset_reason/watchdog_reset tests, which have e.g. 50~100 reset delay tolerance.
Original implementation doesn't enable watchdog reset in pieces of cascaded timeout, except the last one. This is to guarantee re-configuration can be in time, but in interrupt disabled scenario e.g. Hard Fault, watchdog reset can cease to be effective.
This change enables watchdog reset all the way of cascaded timeout. With trade-off, guaranteed watchdog reset function is more significant than re-configuration in time.
Chun-Chieh Li
Martin Kojtal
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