qcom_defconfig: enable fastboot

qcom_defconfig: enable fastboot

Enable fastboot support over USB, using MMC as the backend. This will be
the internal eMMC on devices that have it, or the sdcard slot on devices
with UFS (if available).

We don't use a fixed address for the fastboot buffer because it's
allocated at runtime per-board. Entering fastboot mode should be done by
executing "run fastboot" or manually running:

fastboot -l $fastboot_addr_r usb 0

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250324-sdm845-fixes-fastboot-v1-4-d177a10f336d@linaro.org
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>