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