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.