drivers: cc: mcux: Fix incorrect clock source of FlexSPI2

The clock control mcux rev2 returns FlexSPI1 clock rate when FlexSPI2
clock rate is requested.

Signed-off-by: Martin Stumpf <martin.stumpf@vected.de>

Author: mstumpf-vected

drivers/clock_control/clock_control_mcux_ccm_rev2.c

@@ -198,10 +198,8 @@ static int mcux_ccm_get_subsys_rate(const struct device *dev,
 
 #ifdef CONFIG_MEMC_MCUX_FLEXSPI
 	case IMX_CCM_FLEXSPI_CLK:
-		clock_root = kCLOCK_Root_Flexspi1;
-		break;
 	case IMX_CCM_FLEXSPI2_CLK:
-		clock_root = kCLOCK_Root_Flexspi2;
+		clock_root = kCLOCK_Root_Flexspi1 + instance;
 		break;
 #endif
 #ifdef CONFIG_COUNTER_NXP_PIT