arm64: add macros for common adrp usages

The adrp instruction is mostly used in combination with either
an add, a ldr or a str instruction with the low bits of the
referenced symbol in the 12-bit immediate of the followup
instruction.

Introduce the macros adr_l, ldr_l and str_l that encapsulate
these common patterns.

Tested-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>