mx51: Fix USB PHY clocks

mx51: Fix USB PHY clocks

The i.MX51 has a single USB PHY clock, while the i.MX53 has two. These 3 clocks
have different clock gate control bit-fields.

The existing code was correct only for i.MX53, so this patch fixes the i.MX51
use case.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Jana Rapava <fermata7@gmail.com>
Cc: Wolfgang Grandegger <wg@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>