TCPCI: add ALERT_NONE
This adds the TCPC_REG_ALERT_NONE constant to explicitly checking if the
alert register has no alerts pending.
BRANCH=none
TEST=buildall passes
#define TCPC_REG_PD_INT_REV 0xa
#define TCPC_REG_ALERT 0x10
+#define TCPC_REG_ALERT_NONE 0x0000
#define TCPC_REG_ALERT_MASK_ALL 0xffff
#define TCPC_REG_ALERT_VENDOR_DEF BIT(15)
#define TCPC_REG_ALERT_ALERT_EXT BIT(14)
