arm64: io: Use asm-generic high level MMIO accessors

Remove custom arm64 MMIO accessors read{b,w,l,q} and their relaxed
versions in support to use asm-generic defined accessors. Also define
one set of IO barriers (ar/bw version) used by asm-generic code to
override the arm64 specific variants.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sai Prakash Ranjan <quic_saipraka@quicinc.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>