powerpc/configs: Disable /dev/port in skiroot defconfig

While reviewing lockdown patches, I discovered that we still enable
/dev/port (CONFIG_DEVPORT) in skiroot.

/dev/port is used for old x86 style IO accesses. It's set up in
drivers/char/mem.c, and is only created if arch_has_dev_port() returns
true. Per arch/powerpc/include/asm/io.h, on PPC64 with PCI, this is
only true if there's a legacy ISA bridge.

Even if a system has a legacy ISA bridge installed, we have no
business accessing it in skiroot.

Deselect CONFIG_DEVPORT for skiroot.
Signed-off-by: Daniel Axtens <dja@axtens.net>
[mpe: Incorporate emailed comments into the change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190627053008.29315-1-dja@axtens.net

arch/powerpc/configs/skiroot_defconfig

@@ -213,6 +213,7 @@ CONFIG_IPMI_WATCHDOG=y
 CONFIG_HW_RANDOM=y
 CONFIG_TCG_TPM=y
 CONFIG_TCG_TIS_I2C_NUVOTON=y
+# CONFIG_DEVPORT is not set
 CONFIG_I2C=y
 # CONFIG_I2C_COMPAT is not set
 CONFIG_I2C_CHARDEV=y