Initial Realtek MST i2c_spi support

This spi master allows for programming of a Realtek RTD2142
MST with external SPI flash chip routed via its internal i2c
transport mechanism.

BUG=b:152558985,b:148745673
BRANCH=none
TEST=echo "00000000:0004ffff fw" > layout && \
  flashrom -p realtek_mst_i2c_spi:bus=8 -l layout -i fw:dump.bin -r && \
  flashrom -p realtek_mst_i2c_spi:bus=8 -l layout -i fw:dump.bin -w && \
  flashrom -p realtek_mst_i2c_spi:bus=8 --flash-size && \
  flashrom -p realtek_mst_i2c_spi:bus=8 --flash-name

Change-Id: I892e0be776fe605e69fb39c77abf3016591d7123
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/40667


Reviewed-by: Edward Hill <ecgh@chromium.org>
Reviewed-by: Shiyu Sun <sshiyu@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>