pinctrl: meson: Rename REG_* to MESON_REG_*

Currently compilation test fails on x86 due to name collision. The usual
way to fix that is to move both conflicting parts to their own namespaces.

Rename REG_* to MESON_REG_* as a prerequisite for enabling COMPILE_TEST.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>