am33xx: board: Call spl_early_init() to support sdram_init()

With driver model enabled in SPL, sdram_init() requires device tree
and malloc to be initialized.
Therefore call spl_early_init() in early_system_init().

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>