mips: vcoreiii: Ajust CONFIG_SYS_MEMTEST_END

This patch ajust CONFIG_SYS_MEMTEST_END for vcoreiii-based systems to
avoid overwriting the relocated u-boot. The former setting was too
agressive with networking etc. enabled on some platforms.

Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>