Add PPC4xx-HCU4 and HCU5 boards: READMEs

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+HCU4 Configuration Details
+
+Memory Bank 0 -- Flash chip
+---------------------------
+
+0xfff00000 - 0xffffffff
+
+The flash chip is really only 512Kbytes, but the high address bit of
+the 1Meg region is ignored, so the flash is replicated through the
+region. Thus, this is consistent with a flash base address 0xfff80000.
+
+The placement at the end is to be consistent with reset behavior,
+where the processor itself initially uses this bus to load the branch
+vector and start running.
+
+On-Chip Memory
+--------------
+
+0xf4000000 - 0xf4000fff
+
+The 405GPr includes a 4K on-chip memory that can be placed however
+software chooses. I choose to place the memory at this address, to
+keep it out of the cachable areas.
+
+
+Internal Peripherals
+--------------------
+
+0xef600300 - 0xef6008ff
+
+These are scattered various peripherals internal to the PPC405GPr
+chip.
+
+Chip-Select 2: Flash Memory
+---------------------------
+
+0x70000000
+
+Chip-Select 3: CAN Interface
+----------------------------
+0x7800000
+
+
+Chip-Select 4: IMC-bus standard
+-------------------------------
+
+Our IO-Bus (slow version)
+
+
+Chip-Select 5: IMC-bus fast (inactive)
+--------------------------------------
+
+Our IO-Bus (fast, but not yet use)
+
+
+Memory Bank 1 -- SDRAM
+-------------------------------------
+
+0x00000000 - 0x1ffffff   # Default 32 MB

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