powerpc: drop HPCD/MPC8610 evaluation platform support

This evaluation platform was essentially a single core 8641 with
integrated graphics/display support - in an effort to reduce chip count
on kiosk and similar applications.

Compared to other evaluation platforms considered for removal in other
recent commits, this platform was relatively rare.  Unlike all the other
10+ platforms, I couldn't find any documentation on it - just a link to
downloading the 2007 era BSP in "LTIB" format as was done back then.

With all that in mind, it seems prudent to remove it here in 2023.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
[mpe: Drop stale reference to MPC8610_HPCD in 86xx/Kconfig]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230225201318.3682-4-paul.gortmaker@windriver.com