ARM: keystone2: Convert BOOTBITMASK to static inline function

BOOTBITMASK is almost impossible to decode, so convert it into a simpler
static line functions of equivalent solution.

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>