powerpc/boot: Fix CONFIG_PPC_MPC52XX references

[ Upstream commit e5eff89657e72a9050d95fde146b54c7dc165981 ]

Commit 866bfc75 ("powerpc: conditionally compile platform-specific
serial drivers") made some code depend on CONFIG_PPC_MPC52XX, which
doesn't exist.

Fix it to use CONFIG_PPC_MPC52xx.

Fixes: 866bfc75 ("powerpc: conditionally compile platform-specific serial drivers")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-7-mpe@ellerman.id.au


Signed-off-by: Sasha Levin <sashal@kernel.org>