ARM: rpi: update memory layout env. var. documentation

Update rpi-common.h's documentation that describes the rationale for
choosing various addresses for standardized variables used by boot
scripts. This comment was correct when written, but not updated when some
of the values were changed.

Fixes: 14006a56 ("rpi: set fdt_addr_r to 0x00000100 to match default
...device_tree_address")
Cc: Jonathan Liu <net147@gmail.com>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>