device_mmio: Fixed initialization of anonymous union on legacy compilers

An anonymous union is a feature of the C language which allows a union
declared within a struct to be declared without a name:

    struct foo {
        union {
            int a;
            char b;
        };
    };

A struct containing an anonymous union may be initialized in either of the
following ways:

    /* Form A */
    struct foo test = {
        .a = 1234
    };

    /* Form B */
    struct foo test2 = {
        { .a = 1234 }
    };

Before the release of C11 (which became available in GCC 4.7.1), anonymous
unions were only supported by GNU extensions to the C language: GNU89 or
GNU99.

Support for Form A of anonymous union initialization was added in GCC
4.6.1. Older version only support Form B of the above example.

This patch improves compatiblity with legacy versions of GCC by converting
an instance of anonymous struct initialization from Form A to Form B.

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>

include/zephyr/sys/device_mmio.h

@@ -264,7 +264,7 @@ struct z_device_mmio_rom {
  * @param node_id DTS node_id
  */
 #define DEVICE_MMIO_ROM_INIT(node_id) \
-	._mmio = Z_DEVICE_MMIO_ROM_INITIALIZER(node_id)
+	{ ._mmio = Z_DEVICE_MMIO_ROM_INITIALIZER(node_id) }
 
 /**
  * @def DEVICE_MMIO_MAP(device, flags)