Timer FRC1 & FRC2 registers

Mostly determined from reverse engineering and poking around.

Includes first "experiments" program with random bits and pieces for
poking at registers, may be useful to keep in source control but not
useful for writing actual programs.

core/include/esp/cpu.h

@@ -16,10 +16,16 @@ typedef enum {
     INUM_SPI = 2,
     INUM_GPIO = 4,
     INUM_UART = 5,
-    INUM_MAX = 6,
+    INUM_MAX = 6, /* in some places this is documented as timer0 CCOMPARE0 interrupt */
     INUM_SOFT = 7,
     INUM_WDT = 8,
-    INUM_FRC_TIMER1 = 9,
+    INUM_TIMER_FRC1 = 9,
+
+    /* FRC2 default handler. Configured by sdk_ets_timer_init, which
+       runs as part of default libmain.a startup code, assigns
+       interrupt handler to sdk_vApplicationTickHook+0x68
+     */
+    INUM_TIMER_FRC2 = 10,
 } xt_isr_num_t;
 
 #endif