arm: spear: Support returning to BootROM

arm: spear: Support returning to BootROM

Implement the weak board_return_to_bootrom() function so that when
enabling the spl_bootrom.c driver, one can make use of usbboot on
spear platforms. All necessary information to return to the BootROM
are stored in the BootROM's stack. The SPL stack pointer is reset so
we save the BootROM's stack pointer into the SPL .data section.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>