mtd: maps: physmap-core: fix flash size larger than 32-bit

mtd-ram can potentially be larger than 4GB. get_bitmask_order() uses
fls() that is not guaranteed to work with values larger than 32-bit.
Specifically on aarch64 fls() returns 0 when all 32 LSB bits are clear.
Use fls64() instead.

Fixes: ba32ce95 ("mtd: maps: Merge gpio-addr-flash.c into physmap-core.c")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/9fbf3664ce00f8b07867f1011834015f21d162a5.1707388458.git.baruch@tkos.co.il