mx25l: Add support for Integrated Silicon Solution is25wp256

This is used for the on-board flash on the HiFive Unmatched board.

Reviewed by:	#riscv, jrtc27
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D31562

sys/dev/flash/mx25l.c

@@ -152,6 +152,9 @@ static struct mx25l_flash_ident flash_devices[] = {
 	/* GigaDevice */
 	{ "gd25q64",	0xc8, 0x4017, 64 * 1024, 128, FL_ERASE_4K },
 	{ "gd25q128",	0xc8, 0x4018, 64 * 1024, 256, FL_ERASE_4K },
+
+	/* Integrated Silicon Solution */
+	{ "is25wp256",	0x9d, 0x7019, 64 * 1024, 512, FL_ERASE_4K | FL_ENABLE_4B_ADDR},
 };
 
 static int