mfd: rz-mtu3: Fix COMPILE_TEST build error

When (MFD) RZ_MTU3=m and PWM_RZ_MTU3=y, It hits the below error
aarch64-linux-gnu-ld: rz-mtu3.c:(.text+0x544): undefined reference to `mfd_remove_devices'

Fix this issue by selecting MFD_CORE.
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Closes: https://lore.kernel.org/linux-pwm/20230718090023.wo6m6ffzaifgctkj@pengutronix.de/Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20230718134314.118333-1-biju.das.jz@bp.renesas.comSigned-off-by: Lee Jones <lee@kernel.org>

drivers/mfd/Kconfig

@@ -1373,6 +1373,7 @@ config MFD_SC27XX_PMIC
 config RZ_MTU3
 	tristate "Renesas RZ/G2L MTU3a core driver"
 	depends on (ARCH_RZG2L && OF) || COMPILE_TEST
+	select MFD_CORE
 	help
 	  Select this option to enable Renesas RZ/G2L MTU3a core driver for
 	  the Multi-Function Timer Pulse Unit 3 (MTU3a) hardware available