common/board_r: Move blkcache_init call earlier in the boot sequence

blkcache_init manually relocates blkcache list pointers when
CONFIG_NEEDS_MANUAL_RELOC is enabled. However, it is called very late in
the boot sequence, which could be a problem if previous boot calls execute
blkcache operations with the non-relocated pointers. For example, mmc is
initialized earlier and might call blkcache_invalidate (in
mmc_select_hwpart()) when trying to load the environment from mmc via
env_load().

To fix this issue, move blkcache_init boot call earlier, before mmc gets
initialized.

Acked-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Tested-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>