ppc/ppc405: Rework ppc_40x_timers_init() to use a PowerPCCPU

This is a small cleanup to ease reading. It includes the removal of a
check done on the returned value of g_malloc0(), which can not fail.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211222064025.1541490-6-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220103063441.3424853-7-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>