generic: add MikroTik hw bit description

An e-mail response from MikroTik contained a minimal overview
on hardware option bits which mentioned a currently unknown bit.

While not being too detailed what it does, add it for documentation purposes.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

target/linux/generic/files/drivers/platform/mikrotik/rb_hardconfig.c

@@ -61,6 +61,7 @@
 #define RB_HW_OPT_HAS_VOLTAGE		BIT(1)
 #define RB_HW_OPT_HAS_USB		BIT(2)
 #define RB_HW_OPT_HAS_ATTINY		BIT(3)
+#define RB_HW_OPT_PULSE_DUTY_CYCLE	BIT(9)
 #define RB_HW_OPT_NO_NAND		BIT(14)
 #define RB_HW_OPT_HAS_LCD		BIT(15)
 #define RB_HW_OPT_HAS_POE_OUT		BIT(16)