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

arch/powerpc/platforms/52xx/lite5200_sleep.S

@@ -56,7 +56,7 @@ lite5200_low_power:
 	/*
 	 * save stuff BDI overwrites
 	 * 0xf0 (0xe0->0x100 gets overwritten when BDI connected;
-	 *   even when CONFIG_BDI* is disabled and MMU XLAT commented; heisenbug?))
+	 *   even when CONFIG_BDI_SWITCH is disabled and MMU XLAT commented; heisenbug?))
 	 * WARNING: self-refresh doesn't seem to work when BDI2000 is connected,
 	 *   possibly because BDI sets SDRAM registers before wakeup code does
 	 */