Change MIPS_INTRNG to INTRNG in MEDIATEK_BASE config

Revision 298068 changed MIPS_INTRNG and ARM_INTRNG to simply INTRNG.
MEDIATEK_BASE config was missed by this revision, so we change
MIPS_INTRNG to INTRNG here.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5978