add dna command to print out efuse dna registers

[ljones-source]

add a dna command to u-boot.
this makes it easy to find the 'fpga id', which is called the efuse dna 0,1, and 2 registers (three 32 bit registers). This requires u-boot to be patched to allow for the efuse registers to be "read". u-boot does not directly access them, it needs to ask the PMUFW to fetch them, but does not have the permission to do this by default.

ZynqMP> dna
40000000019675c745312205

without the dna command (still need permissions):

ZynqMP> zynqmp mmio_read 0xffcc100c
mmio read value at 0xffcc100c = 0x45312205
ZynqMP> zynqmp mmio_read 0xffcc1010
mmio read value at 0xffcc1010 = 0x19675c7
ZynqMP> zynqmp mmio_read 0xffcc1014
mmio read value at 0xffcc1014 = 0x40000000

The dna command is essentially just a shortcut for these three commands.