Documentation: Rename PPC_FSL_BOOK3E to PPC_E500

CONFIG_PPC_FSL_BOOK3E is redundant with CONFIG_PPC_E500.

Rename it so that CONFIG_PPC_FSL_BOOK3E can be removed later.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d3d42b395c09e66b0705fda1e51779f33e13ac38.1663606876.git.christophe.leroy@csgroup.eu

Documentation/admin-guide/kernel-parameters.txt

@@ -3626,7 +3626,7 @@
 			(bounds check bypass). With this option data leaks are
 			possible in the system.
 
-	nospectre_v2	[X86,PPC_FSL_BOOK3E,ARM64] Disable all mitigations for
+	nospectre_v2	[X86,PPC_E500,ARM64] Disable all mitigations for
 			the Spectre variant 2 (indirect branch prediction)
 			vulnerability. System may allow data leaks with this
 			option.