serial: move sbi_dbcn_available to .data section

serial: move sbi_dbcn_available to .data section

U-Boot SPL loads the device-tree directly behind main U-Boot overlapping
the .bss section. reserve_fdt() is called in board_init_f() to relocate the
device-tree to a safe location.

Debug UARTs are enabled before board_init_f(). With sbi_dbcn_available in
the .bss section the device-tree is corrupted when _debug_uart_init() is
called in the SBI serial driver. Move the variable to the .data section.

Link: https://bugs.launchpad.net/ubuntu/+source/u-boot/+bug/2054091
Fixes: dfe08374943c ("risc-v: implement DBCN based debug console")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Conor Dooley <conor.dooley@microchip.com>