Documentation: bindings: Modify Neutrino RMII option to u32

Modify description of the RMII mode option in the Neutrino
bindings to make it clear that it is a u32 option with
possible values of <0> for disabled and <1> for enabled.

Change-Id: Ic345f2dd95353cde75b5d81d9812b7217f9df054
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>