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>