omap: Fix AM335x build with enabled fastboot flash

omap: Fix AM335x build with enabled fastboot flash

When enabling CONFIG_FASTBOOT_FLASH in am335x_boneblack_defconfig, next
build errors and warnings occur:

    arch/arm/mach-omap2/utils.c: In function ‘omap_set_fastboot_cpu’:
    arch/arm/mach-omap2/utils.c:26:16: warning: implicit declaration of
              function ‘omap_revision’ [-Wimplicit-function-declaration]
              u32 cpu_rev = omap_revision();
                            ^~~~~~~~~~~~~
    arch/arm/mach-omap2/utils.c:29:7: error: ‘DRA762_ES1_0’ undeclared
              (first use in this function)

Include asm/omap_common.h explicitly to avoid those.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>