iotrace: Finish migrating this to Kconfig

When I migrated this to Kconfig in commit 68e54040cc ("sandbox: Move
CONFIG_IO_TRACE to Kconfig") I didn't look hard enough for other
details. As explained in the README, this is valid for ARM too. So start
by making this be a prompted question and CMD_IOTRACE depend on IO_TRACE
being enabled.  Next, migrate the information out of README and in to
the appropriate help text for existing options in Kconfig. Finally, make
this option be default y on SANDBOX but not selected as it's valid to
build without it.

Signed-off-by: Tom Rini <trini@konsulko.com>

README

@@ -300,28 +300,6 @@ The following options need to be configured:
 		Note that if the GPIO device uses I2C, then the I2C interface
 		must also be configured. See I2C Support, below.
 
-- I/O tracing:
-		When CONFIG_IO_TRACE is selected, U-Boot intercepts all I/O
-		accesses and can checksum them or write a list of them out
-		to memory. See the 'iotrace' command for details. This is
-		useful for testing device drivers since it can confirm that
-		the driver behaves the same way before and after a code
-		change. Currently this is supported on sandbox and arm. To
-		add support for your architecture, add '#include <iotrace.h>'
-		to the bottom of arch/<arch>/include/asm/io.h and test.
-
-		Example output from the 'iotrace stats' command is below.
-		Note that if the trace buffer is exhausted, the checksum will
-		still continue to operate.
-
-			iotrace is enabled
-			Start:  10000000	(buffer start address)
-			Size:   00010000	(buffer size)
-			Offset: 00000120	(current buffer offset)
-			Output: 10000120	(start + offset)
-			Count:  00000018	(number of trace records)
-			CRC32:  9526fb66	(CRC32 of all trace records)
-
 - Timestamp Support:
 
 		When CONFIG_TIMESTAMP is selected, the timestamp