mtd: spi-nor: core: fix implicit declaration warning

spi-nor/core.c needs to include linux/delay.h,
or it would raise below compile warning:

drivers/mtd/spi-nor/core.c: In function ‘spi_nor_soft_reset’:
drivers/mtd/spi-nor/core.c:2779:2: error: implicit declaration of function ‘usleep_range’ [-Werror=implicit-function-declaration]
 2779 |  usleep_range(SPI_NOR_SRST_SLEEP_MIN, SPI_NOR_SRST_SLEEP_MAX);
      |  ^~~~~~~~~~~~

Fixes: d73ee753 ("mtd: spi-nor: core: perform a Soft Reset on shutdown")
Signed-off-by: Zeng Heng <zengheng4@huawei.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20220923031457.56103-1-zengheng4@huawei.com

drivers/mtd/spi-nor/core.c

@@ -15,6 +15,7 @@
 #include <linux/math64.h>
 #include <linux/sizes.h>
 #include <linux/slab.h>
+#include <linux/delay.h>
 
 #include <linux/mtd/mtd.h>
 #include <linux/of_platform.h>