AM3517: Changed default clock rate for AM3517

AM3517: Changed default clock rate for AM3517

Changed #define MPU_M_13_ES2 from 0x1F4 to 0x258, this allows
the AM3517 to boot up at 600MHz instead of 500 MHz

Signed-off-by: Schuyler Patton <spatton@ti.com>
CC: Tom Rini <tom.rini@gmail.com>
CC: Simon Schwarz <simonschwarzcor@gmail.com>
CC: Stefano Babic <sbabic@denx.de>