ARM: omap2plus_defconfig: make PCF857x built-in

One of the lines from PCF857x is connected to the vdd line of MMC1
in DRA74x and DRA72x EVMs and is modelled as a regulator. If PCF857x
is not made as built-in, the regulator_get in omap_hsmmc fails making
it difficult to use MMC1 as rootfs.

Make PCF857x built-in.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>