board: stmark2: fix clock value

Fix totally blank console at boot, clock value must be decimal,
as for the 30Mhz external crystal.

Fixes: 26e5944e ("stmark2: Migrate CONFIG_SYS_EXTRA_OPTIONS to Kconfig")
Signed-off-by: Angelo Dureghello <angelo@kernel-space.org>
Reviewed-by: Tom Rini <trini@konsulko.com>