target/ppc: powerpc_excp: Set alternate SRRs directly

target/ppc: powerpc_excp: Set alternate SRRs directly

There are currently only two interrupts that use alternate SRRs, so
let them write to them directly during the setup code.

No functional change intended.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20211229165751.3774248-2-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>