bsp/csb336: Fix MMU _ttbl_base location to not overlap with exception vectors.

The initial region (64 bytes) of SDRAM RTEMS image is remapped
to provide overlay of the initial/ROM exceptions table.
This area cannot be used for MMU mapping table. Different correctly
aligned block has to be used for MMU table. Remapping of SDRAM
(address 0x08200000) to address 0 is supported only on 1 MB block
granularity and that is why SDRAM_VEC area has to be 1 MB aligned too
but unused part of remapped region can be freely used for other
purposes (as MMU tables).

Signed-off-by: Pavel Pisa <ppisa@pikron.com>
bsp/csb336: implement bsp_interrupt_vector_enable/disable.

Signed-off-by: Pavel Pisa <ppisa@pikron.com>