esp/wdt_regs.h: Add "current value" register.

WDT is countdown timer. Current value is accessible via VAL register. At
this time it's unclear if it's RO or RW (common sense says it shoul be RO).

Source: looking at the WDT registers on a running chip.

core/include/esp/wdt_regs.h

@@ -25,7 +25,9 @@ struct WDT_REGS {
     uint32_t volatile CTRL;        // 0x00
     uint32_t volatile REG1;        // 0x04
     uint32_t volatile REG2;        // 0x08
-    uint32_t volatile _unused[2];  // 0x0c - 0x10
+    // Current value, decrementing
+    uint32_t volatile VAL;         // 0x0c
+    uint32_t volatile _unused[1];  // 0x10
     uint32_t volatile FEED;        // 0x14
 };