system/utils/README : Update heapinfo cmd information
system/utils/README : Update heapinfo cmd information

1. heapinfo supports -e, -r and -k options, but there was no description in README.
and for using heapinfo cmd, CONFIG_DEBUG_MM_HEAPINFO is mandantory, so changing the explanation for enabling the heapinfo cmd.
2. CONFIG_ENABLE_HEAPINFO is changed to CONFIG_ENABLE_HEAPINFO_CMD.
3. fix typo for irqinfo

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