ARM: dts: am437x-idk-evm: Fix incorrect OPP node names

The commit 337c6c9a ("ARM: dts: am437x-idk-evm: Disable
OPP50 for MPU") adjusts couple of OPP nodes defined in the
common am4372.dtsi file, but used outdated node names. This
results in these getting treated as new OPP nodes with missing
properties.

Fix this properly by using the correct node names as updated in
commit b9cb2ba7

 ("ARM: dts: Use - instead of @ for DT OPP
entries for TI SoCs").

Reported-by: Roger Quadros <rogerq@ti.com>
Fixes: 337c6c9a

 ("ARM: dts: am437x-idk-evm: Disable OPP50 for MPU")
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>