mx28: Fix typo in POWER_MINPWR_VBG_OFF

The POWER_MINPWR_VBG_OFF bit in mx28 power supply block is not called
POWER_MINPWR_FBG_OFF, but POWER_MINPWR_VBG_OFF. Correct the name in the
header file.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>