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

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

The commit 0a615df59005 ("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common am4372.dtsi file, but used outdated node names.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 daf13cee487d ("ARM: dts: Use - instead of @ for DT OPP
entries for TI SoCs").

Reported-by: Roger Quadros <rogerq@ti.com>
Fixes: 0a615df59005 ("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>