powerpc/powernv: make symbol 'mpipl_kobj' static

The sparse tool complains as follows:

arch/powerpc/platforms/powernv/opal-core.c:74:16: warning:
 symbol 'mpipl_kobj' was not declared.

This symbol is not used outside of opal-core.c, so marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210409063855.57347-1-cuibixuan@huawei.com