mmc: cavium-octeon: Use proper GPIO name for power control

The devm_gpiod_get_optional() function appends a "-gpios" to the
string passed to it, so if we want to find the "power-gpios" signal,
we must pass "power" to this function.

Fixes: 01d95843 ("mmc: cavium: Add MMC support for Octeon SOCs.")
Signed-off-by: David Daney <david.daney@cavium.com>
[jglauber@cavium.com: removed point after subject line]
Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>