UCB RISCV: Switch to DYNAMIC_CBMEM

Change-Id: Iaaf68fd19f7b9a5b6849fffde3a9c68cb7862367
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7619
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>