egl: Use reserved values for eglGetFrameTimestamps

Also update opengl/specs/README so it's clear which blocks
of values are already reserved.

Bug: 36887025
Test: --deqp-case=dEQP-EGL*get_frame_timestamps*
Change-Id: I2cd83553cc475fdf3f8cdfadf2eaf32abc8a7c09