arm64: zynqmp: Decouple MMIO accessors from firmware

zynqmp_mmio_read() and zynqmp_mmio_write() selected between direct MMIO
and the firmware (PM_MMIO_READ/WRITE) interface with an in-function
IS_ENABLED(CONFIG_ZYNQMP_FIRMWARE) / current_el() check. Generic arch
code should not carry firmware-specific ifdefs, and with SCMI the access
method changes again.

Split the accessors like the multiboot and bootmode hooks: the weak
default in arch/arm/mach-zynqmp does the direct MMIO access (used in SPL,
at EL3 and when no firmware is present), while firmware-zynqmp.c provides
a strong definition that issues the firmware call and falls back to the
direct access in SPL/EL3 where the SMC path is unavailable. The raw MMIO
primitives zynqmp_mmio_rawread() and zynqmp_mmio_rawwrite() are exported
for the shared fallback, and the read-modify-write helper now uses the
raw read instead of routing through the firmware-aware accessor.

The firmware-vs-MMIO decision is selected at link time, so adding SCMI
later only requires a third strong definition with no changes to generic
code.

Signed-off-by: Michal Simek <[email protected]>
Link: https://patch.msgid.link/d532df144d2c8e34be835bad6d0de3b26befdf01.1782219202.git.michal.simek@amd.com

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