realtek_mst_i2c_spi.c: Add allow-brick=yes programmer param

Currently i2c programmers do not have a safe allow listing
mechanism via board_enable to facilitate fully qualified
chip detection.

Since i2c addresses alone can overlap a user may make the mistake
of using the wrong programmer. Although unlikely, it is within the
realm of possibility that a user could accidently somehow program
another chip on their board.

Change-Id: Ifb303989fdb67f7267002bd0425f3d050450ec93
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/65545
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

flashrom.8.tmpl

@@ -1554,6 +1554,9 @@ hub when beginning operation to put it into ISP mode.
 the MST hub on programming completion, causing it to exit ISP mode and to
 reload its own firmware from flash.
 
+\fBallow-brick\fP defaults to no, however must be set explicitly to "yes"
+to allow the driver to run if you are sure you have a MST chip.
+
 The hub must be in ISP mode for SPI flash access to be possible, so it is
 usually only useful to disable \fBenter-isp\fP if an earlier invocation avoided
 resetting it on completion. For instance, to erase the flash and