pmdomain: mediatek: mtk-mfg: select MAILBOX in Kconfig

The mtk-mfg pmdomain driver calls common mailbox framework functions. If
the common mailbox framework is not selected in the kernel's
configuration, the build runs into a linker error, as the symbols are
absent.

The hardware mailbox Kconfig system, MAILBOX, has no dependencies of its
own. It's therefore safe to "select" it rather than use "depend on".

Declare this "select" dependency in the Kconfig for the driver.

Fixes: 1ff1f0db6aec ("pmdomain: mediatek: Add support for MFlexGraphics")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Nicolas Frattaroli <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/pmdomain/mediatek/Kconfig

···
 	depends on PM
 	depends on OF
 	depends on COMMON_CLK
+	select MAILBOX
 	select PM_GENERIC_DOMAINS
 	imply MTK_GPUEB_MBOX
 	help