configs: restrict am62ax wakup SPL size

In its current form, the am62a's wakeup SPL is fairly small, however
this will not remain as more boot modes are eventually added. To protect
us from overflowing our ~256k of HSM SRAM, add limits and check during
the wakeup SPL build.

Signed-off-by: Bryan Brattlof <bb@ti.com>