watchdog: booke_wdt: Replace PPC_FSL_BOOK3E by PPC_E500

CONFIG_PPC_FSL_BOOK3E is redundant with CONFIG_PPC_E500.

Replace it so that CONFIG_PPC_FSL_BOOK3E can be removed later.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/01a9132d51d3d8d9c74576d3da4d9d1fa5a88bde.1663606876.git.christophe.leroy@csgroup.eu