nios2: trim CONFIG_SYS_MEMTEST_END

Trim CONFIG_SYS_MEMTEST_END location.

CONFIG_SYS_MONITOR_LEN
  Reserving 256k for U-Boot at: d7fc0000
CONFIG_ENV_SIZE
CONFIG_SYS_MALLOC_LEN
  Reserving 256k for malloc() at: d7f80000
0x10000 for the rest
  Reserving 68 Bytes for Board Info at: d7f7ffbc
  Reserving 208 Bytes for Global Data at: d7f7feec
  Reserving 12000 Bytes for FDT at: d7f7d00c
  Stack

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>