boards: microchip: skip flash based CI tests for PolarFire SoC

Microchip's PolarFire SoC has a system controller that utilizes an
external spi flash for performing "auto update" of the FPGA design. This
is not a feauture fully integrated into Zephyr as of yet, so in an
effort to not constantly fail the CI, add flash tag to the ignore list.

Signed-off-by: Conor Paxton <conor.paxton@microchip.com>