board: samsung: reserve memory for the secure firmware

Since more than one board requires memory reservation
for the secure firmware, the reservation code can be
made in a common code.
Now, to reserve some part of the the last bank,
board config should define:
- CONFIG_TZSW_RESERVED_DRAM - len in bytes
- CONFIG_NR_DRAM_BANKS - number of memory banks

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Akshay Saraswat <akshay.s@samsung.com>
Cc: Hyungwon Hwang <human.hwang@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>