Add support to RamDump.virt_to_phys for automatically looking up
addresses of symbols when a string is passed in. This will simplify code
that uses the RamDump.read_* routines.

Old way:

    mydata_addr = ramdump.addr_lookup('data_var')
    mydata2_addr = ramdump.addr_lookup('data2_var')

    mydata = ramdump.read_word(mydata_addr)
    mydata2 = ramdump.read_word(mydata2_addr)

New way:

    mydata = ramdump.read_word('data_var')
    mydata2 = ramdump.read_word('data2_var')

Of course, the "Old way" will still work.

Change-Id: I0c006f5135b48f8385d2107d9b9d2fe767a266b0