arm: baltos: active mPCIe slot

Baltos devices provide a mPCIe slot, whose power is turned off by
default. This patch activates mPCIe slot in U-Boot, so that for example
GSM modem can be already available in user space.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>