mips_malta: Clean up definition of flash memory size somewhat

mips_malta: Clean up definition of flash memory size somewhat

pflash_cfi01_register() takes a size in bytes, a block size in bytes
and a number of blocks.  mips_malta_init() passes BIOS_SIZE, 65536,
FLASH_SIZE >> 16.  Actually consistent only because BIOS_SIZE (defined
in include/hw/mips/bios.h as (4 * MiB)) matches FLASH_SIZE (defined
locally as 0x400000).  Confusing all the same.

Pass FLASH_SIZE instead of BIOS_SIZE.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Aleksandar Rikalo <arikalo@wavecomp.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190308094610.21210-14-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>