ARM: dts: AM33xx: Fix properties on gpmc node

The gpmc driver is actually looking for "gpmc,num-cs" and
"gpmc,num-waitpins" properties in DT. The binding doc also states
this.
Correct the properties in the dts to provide the right values for the
gpmc driver.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>