arm64: Simplify NR_FIX_BTMAPS calculation

We choose NR_FIX_BTMAPS such that each slot (NR_FIX_BTMAPS * PAGE_SIZE)
can address 256K.

Use division to derive NR_FIX_BTMAPS rather than defining it for each
page size.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>