Currently, s_mxic_set_required_regs() lacks checking for
CONFIG_SPI_FLASH_SUPPORT_MXIC_OPI_CHIP. And this causes a defined but not
used warning when MXIC flash driver is disabled in project config. So add
a #if check for this to supress warning.
Signed-off-by: Shengyu Qu <wiagn@4d2.org>