wifi: mwifiex: Add missing compatible string for SD8787

Commit e3fffc1f ("devicetree: document new marvell-8xxx and
pwrseq-sd8787 options") documented a compatible string for SD8787 in
the devicetree bindings, but neglected to add it to the mwifiex driver.

Fixes: e3fffc1f ("devicetree: document new marvell-8xxx and pwrseq-sd8787 options")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v4.11+
Cc: Matt Ranostay <mranostay@ti.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/320de5005ff3b8fd76be2d2b859fd021689c3681.1674827105.git.lukas@wunner.de