powerpc/8xx: Enhance readability of trap types

This patch makes use of trap types in head_8xx.S

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e1147287bf6f2fb0693048fe8db0298c7870e419.1618847273.git.christophe.leroy@csgroup.eu