imx: mx6ul: using runtime check when configuring PMIC_STBY_REQ

Since MX6ULL select MX6UL, we can not use IS_ENABLED(CONFIG_MX6UL) here,
because this piece code is only for i.MX6UL.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>

arch/arm/cpu/armv7/mx6/soc.c

@@ -356,7 +356,7 @@ int arch_cpu_init(void)
 			set_ahb_rate(132000000);
 	}
 
-	if (IS_ENABLED(CONFIG_MX6UL) && is_soc_rev(CHIP_REV_1_0) == 0) {
+	if (is_mx6ul() && is_soc_rev(CHIP_REV_1_0) == 0) {
 		/*
 		 * According to the design team's requirement on i.MX6UL,
 		 * the PMIC_STBY_REQ PAD should be configured as open