SD card high-speed/low-voltage mode can only be exited by powercycling.
Pi 4s before rev 1.4 lack the power switch required to do this, so
must resort to a global reset that turns off many things, including
SDRAM.
To ensure correct operation, the bootloader checks that the SD I/O
voltage is the expected 3.3V, forcing a power cycle if it isn't.
However, this doesn't take advantage of presence of the dedicated
SD power switch, always forcing a global reset, a consequence of which
can be the loss of SDRAM content - including any ramoops dump of the
crash logs.
Make the bootloader more SD_PWR_ON aware, only triggering a global
reset if one isn't found.
See: https://github.com/raspberrypi/linux/issues/5298
3c822369be