powerpc/52xx: Fix comment about CONFIG_BDI*

There's a comment in lite5200_sleep.S that refers to "CONFIG_BDI*".

This confuses scripts/checkkconfigsymbols.py, which thinks it should
be able to find CONFIG_BDI.

Change the comment to refer to CONFIG_BDI_SWITCH which is presumably
roughly what it was referring to. AFAICS there never has been a
CONFIG_BDI.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-3-mpe@ellerman.id.au